So this is interesting because it seems to be a point of confusion with a lot of people to the point where it got me thinking. It's odd seeing people talking about a table with a bunch of GR-75's milling around not looking right because we see repeatedly in universe that rebel fleets just seem to have a lot of these things and take them into combat (almost every Return of the Jedi scene of the rebel fleet has one or more messing around), but it's also interesting, because nobody seems to know why they're present. Why would you bother bringing an unarmed or negligibly armed transport in your assault fleet? There are definitely some possible explanations. Most sources agree that the GR-75 has two specific traits 1) Readily obtainable in large numbers and 2) modular configuration.

-Command and Control-

It makes sense on some levels to use a GR-75 as a C&C vessel because the modular nature would let you put a lot of very specific equipment on board and that equipment wouldn't necessarily change the profile of the ship i.e. it would still look like an unarmed transport of no importance. Unless the enemy specifically knew it's role in the fleet, it's unlikely to be fired upon when combat ships are available as a target. It could also fit more specialized equipment more easily than a dedicated combat ship because the needs of the vessel itself are much lighter. While this doesn't seem hugely farfetched, I don't completely buy it. We've never really seen a cannon fleet composition that has the C&C being managed from a GR-75. The role is almost exclusively handled by flagships which are usually things like MC-80's i.e. well armed and shielded with plenty of room for support staff. If this is a use, it doesn't seem to be the case for larger fleet engagements.

-Electronic Warfare-

Whether jamming or directly hacking enemy systems, EW does seem like a legitimate use for the platform. My only issue is that hacking would require some pretty short range which makes little sense. An Imperial ship that noticed a GR-75 moving towards it abnormally aggressively could probably spare a few turbolasers to redecorate space with the thing as, while they seem to pretty well armored against starfighter lasers, are just target practice for more powerful weapons. Electronic jamming would at least make sense on some level as the Alliance is pretty fighter dependent, so making it harder to target them with anti-fighter weaponry would be valuable use and wouldn't necessarily put in harm's way to the same degree as offensive hacking would, but I can't see either of these as being anything but niche applications.

-Q-ships or Fire Ships-

Just no. The platform is just too obviously weak for either application to make sense. It can't handle a number of turbolasers to have a meaningful impact and the ship is so poorly suited for ship to ship combat that you'll lose any element of surprise as soon as you plot a course that brings you closer to the enemy because what other reason would you have to do that? You would need so many converted to this role to have a meaningful impact that even with the availability of the transport being so high, this seems unlikely and impractical just because of the time to convert it over. A few self defense laser cannons is a semi-common modification, but that seems to be the closest these ships ever get to direct combat.

-Pocket Carrier-

At first glance, this is a firm "no", but the more I look at this application, the more I think this might actually be the answer (or at least one of them). The reason it looks like a hard "no" at first is that it's just too small to act as a true carrier. Even if you converted all of the internal space to hangars, you couldn't fit more than a handful of starfighters. They could certainly carry unassembled starfighters, but that doesn't help much in the carrier role. However, one thing to consider is that every rebel starfighter that we've seen in cannon is hyperspace capable. X, Y, A and B-Wings all have hyperdrives and, at the time of the galactic civil war, even most of the Z-95's seem to have one. Granted these hyperdrives are short ranged, but this gives an interesting clue for why we might be seeing these dopey flying footballs so often. If you had a starfighter corps that could jump to combat and out of combat without the need for a dedicated carrier, then that would reduce greatly what you would need a carrier for. You would need the carrier itself to re-arm, re-fuel, repair, haul ground crews and other supplies along with the pilot's personal effects. This...is actual something the GR-75 could conceivably do. It easily has the internal space for fuel, ordinance, spare parts (possibly even an unassembled fighter or two), ground crews and equipment and possibly a few tugs for in combat re-arming and refueling. This actually makes way more sense than it should in universe. This would give you the ability to operate squadrons in a carrier-like capacity with marginal investment cost-wise. You would still need somewhere to land, but you wouldn't need a massive a permanent ground base, just a large enough plot of land to plop down the GR-75 and the squadron(s) (and given that repulsor tech seems to be able to allow these starfighters a semi-vertical take off you wouldn't need anything so big as a runway).

This would also make sense as to why we keep seeing these derpy ships floating around in major battles because in these engagements, you would have far more starfighters than carriers in your fleet would be capable of managing on their own, so the extra coordination and potential in combat re-arming and re-fuelling may be necessary to use these extra squadrons. This use also makes sense for the Rebellion as a whole. Getting starfighters looks pretty easy for the Rebels as does getting access to the GR-75's, but capitol ships of any large size seem to be in short supply. Also, even if you could find and operate one of these larger carriers, it may not be what you want. A single large ships is easier to find and hunt down and gives you a vulnerability that the GR-75 doesn't. In fact, the less decentralized nature of these pocket carriers would actually be beneficial for the hit-and-run and raiding tactics of the rebellion. This isn't supported by any cannon, then again neither is literally any use for the GR-75 beyond just carrying stuff, but it at least makes enough sense to me as to be a believable reason as to why these little buggers seem to be everywhere.

They could also be used as lightly armed "Barrage Balloons", preventing small imperial capital ships from pulling up along-side weaker rebellion capital ships such as nebulons (or at least providing a very unwelcoming passage, filled with small lasers and flying, explosive road-bumps.)

Would explain why so many seem to hug the sides of the larger capital ships so closely...

Respectfully. Isn't this what the Fleet Support upgrades describe? Or am I missing a point? With the exception of "Repair Crews", all the upgrades are useful in combat.

If you are talking about what would pass for real life, the U. S. Navy has noncombat electronic warfare ships for instance and that would cover our "Jamming Field", "Comms Net", and "Slicer Tools". I would expect to find resupply ships behind the fleet and out of the combat zone.

If you are talking about "on screen", then speculation is entirely appropriate as I don't remember seeing them do anything noticeable.?

And the Russians have "fishing trawlers"!

They had the models, they were expensive to make, they were put to good use in beefing up the fleets for the movies.

14 minutes ago, n815e said:

They had the models, they were expensive to make, they were put to good use in beefing up the fleets for the movies.

This is the real reason overall. Different size vessels to give scale and a rag tag civilian look. The onus has always been on the expanded content of Star Wars to justify many of the cinematic decisions of the movie makers.

In this case we can surmise that these transports are used as emergency vehicles ala Hoth (maybe also during space battles, picking up escape pods), potentially as pocket carriers or fleet tenders as explained well above by others, and lastly may have served as special platforms for electronic warfare again as explained before.

I would add that the Rebels are constantly on the move with almost no permanent bases. The fleet itself seems to be main base overall after Yavin. We never saw Mon Mothma or the main rebel forces at Hoth so while it may have been intended to be the primary planetary base the Empire was able to find it so soon they barely had time to bring a proper shield online and refit their ships for local conditions.

This speaks to me of the fact that they just need to carry everything around with them at almost all times. Long range communication was dangerous and ran the risk of being detected by the Empire. Having the fleet split may have offered more protection to these types of transports but could have put the whole rebellion at risk. The speed with which they may have had to react to changing circumstances also lends credence to the choice of keeping it all in one large fleet.

Ultimately we don't have a canon reason so you just have to make do with whatever explanation works best for your own interpretation.

https://en.m.wikipedia.org/wiki/Convoy_rescue_ship

Actually, i can see the GR-75 being turned into lightly armed ships, similar to the armed variants of the Higgens boat in WW 2. There were Landing Craft flak variants (8 20mm Oerlikon cannons and 4 Quad 2 pounder "pom-pom" guns), gun variants (2 25-pounder howitzers or 2 4.7" naval guns), rocket variants (1,000 rockets with 5,000 rounds in reserve) and support variants (different types had .50 machineguns, 4" mortars, 40mm, 57mm or 3" guns and rocket launchers).

That's easy OP!!

GR-75 are simply "Target Practice"!

I can see one effective combat refit for GR-75s, and that's to borrow the missile targeting concept used in one of the X-Wing novels: outfit a bunch of light ships with targeting and relay equipment, which is then used to provide targeting telemetry to a larger dedicated missile cruiser or group of cruisers. By providing the missile telemetry to the larger combat ship, the combat ship could launch rockets without needing to get individual firing solutions with its own equipment. So a system could be used to provide firing telemetry for a large group of ships working in concert.

This sort of tactic is already reportedly being used on modern battlefields using modified ground support aircraft like the AC-130 and overhead drones.

For non-combat roles, these would be excellent rescue craft, flak emplacements, and quick support craft for running critical supplies and personnel between ships before major action began. It would also allow critical non-combat personnel and supplies an option to rapidly evacuate before battle is joined.

It is the same reason Carrier Battle Groups in real life have logistics and supply vessels with them. Those GR75s carry fuel, ammo, food, cargo, and all sorts of other stuff that the warships consume over time. Without the supply ships, the fleet would not have nearly the longevity it has.

If nobody else is going to say it... Activation Advantage!

Eckhart's Ladder covered a lot of the same points you do, in a video:

We're rebels, the masters of improvising! Whatever support role we can visualize, the GR-75 can fill!

Haha. In the above video he dismisses "fuel" as a factor in Star Wars.

Poor guy was trumped by the latest movie.

When you have a faction that is as "nomadic" in nature as the Rebels (or rather, they have to be able to adapt that nature at a moment's notice), a cheap and reliable supply and support vessel makes perfect sense.

Them being present in pitched battles may actually have started as a necessity, after all, what's the alternative? Leaving them behind on their own, to be picked off by an enemy that can potentially always spare dedicated support vessels to wipe them out? Compare how often in history the raid of the support train or camp has spelled disaster and just the fear of it has altered battle plans. Alternatively, if you ever played the Rebellion board game, leave some on their own and see what happens.

If they can help in any way, that might then at first just have been a bonus that was forged into multiple, flexible support roles by the crafty pragmatists of the Alliance.

Rebels need anything and everything they can get their hands on. That includes GR75s.

While not combat vessels, GR75s have shields, and have usually been modified to carry at least a minimal compliment of laser cannons. They serve several purposes for the Rebels. They can add additional firepower to the fleet. This can be used to target income fighters, or just to support fire against capital ships. With Imperial forces more focused on larger capital ships designed for combat, these smaller vessels can often be ignored. Beyond that, they can be positioned to act as shields for larger craft. A larger ship loses a section of shields, or has weapons problems along one part of the ship, the GR75s can slide into position to act as a blocking element to protect that part of the ship until it recovers. Their presence also makes it harder for smaller ships and fighters to make attack runs on larger capital ships.

GR75s are also supply and support vessels. They can be used to tow damaged vessels. They could have repair crews ready to launch to damaged ships to help with repairs. They carry fuel and ammo. They can be used to scoop up escape pods to rescue personnel. They likely are used as barracks also. Beyond that, a considerable amount of their ground forces and emergency base materials are located on those GR75s. If a ground operation is needed, you need those troops with you. Take RTJ for example. What if it was determined that the ground forces needed support. They could divert the GR75s to the surface, while the main ships remain in battle.

It still seems risky to bring and use these limited support ships in battle. You're right, it does. But think about how mobile the rebel forces are. They don't have ship yards and repair facilities all over the place. They don't have space ports and weight stations for collecting new supplies or getting repairs done. The GR75s play those roles for the Rebel fleet. Hyperspace is not an instant thing. It takes time. After a battle with the Empire, the Rebels need to jump to a new location, repair and refit, and then jump again. They don't have bases to return to, they don't have enough ships that they can leave some behind to catch up later. They need to preserve everything. If the GR75s weren't with them in the battle, they wouldn't be with them when they jump away after the battle. Or if it's a situation where the Rebels are winning, and at least temporarily occupying a region of space, they need those repair facilities with them.

Well, why run them with the fleet then? Why not hold them back a hundred kilometers or so? Well, what would be protecting them? If they are in the middle of the fleet, they can depend on other ships to protect them. If you held them back, then a squadron or two of tie fighters could be dispatched to harass them. While not ideal to use such craft in a combat situation like the Rebels do, it's the best option they have.

nice ideas... just some are a little contradictory. (if you read it that every GR-75 is used the same way)

these ships are full of troops /supplies/ repair crews

but they are also the same ships we use as mobile fire brakes....

I'd imagine that they wouldn't really attract much attention in combat, at least not when there are 2 liberties and a few squadrons flying at you, priorities go elsewhere. This ability to go under the radar would allow them to act like PBY Catalina's in WWII, recovering injured personnel, bailed out pilots, sailors who went overboard, or those abandoning a sinking vessel.

We never see it in the movies, but even TIE fighters have ejection seats, ships lose atmosphere containment, if you could tractor beam the injured aboard a small quick ship, survival rates would go up, you can transfer the wounded and emergency supplies at the next hyperspace stop after the battle.

But these ships are roomy, carry supplies, and have small crew requirements, and according to wookiepedia, they are armed with 4 twin laser turrets, so they're like a floating AA platform. Not bad in a fleet when you're always outnumbered.