I'm currently looking at Deatwatch Captain's ability, Command and Control. One of the possible choices is:

Improved Tactics—taking time to review data on the Killteam’s
intended target, the Battle-Brother gains insight
into the intentions of the enemy. The Battle-brother may
re-roll a number of Tactics Tests up to his Intelligence
Bonus during the course of the Mission. These Tactics
Tests must be directly related to his Kill-team and their
actions on the Battlefield. Furthermore, this ability allows
all members of the Kill-team to automatically enter squad
mode a number of times equal to the Watch Captain’s
Intelligence Bonus during the Mission.

Now from the description in core, it seems that the uses of Tactics are mainly RP related ('A character can use Tactics at any time when he wants to determine the best military course of action in a combat.').

Are there any other uses that provide actual mechanical benefits with successful Tactics Tests?

Not that I know of, but I would say that knowing the best course of action is quite a bit better than a mechanical benefit.

Unfortunately not. When players want to roll tactics, it can be rather frustrating to figure out a way to give them SOME relevant information without giving them TOO much, and also weighting fluff vs mechanical benefit too.

Tactics is also good if you're a smurf. First Founding Solo Mode gives Ultramarines rerolls in a combat equal to DoS+1 on a Tactics roll.

I believe there's only a game mechanic for Tactics (stealth & recon).

Not that I know of, but I would say that knowing the best course of action is quite a bit better than a mechanical benefit.

The problems with such approach are....quite a few: For starters, I'm probably one of the best tactical mind in my group so the odds of actually getting some insight I haven't figured out on my own are pretty remote. Usually it's just an IC way to justify insight I already have OOC. Which leads to a new problem: where do I draw the line? What moves are common sense and what moves require a tactics roll?

Tactics is also good if you're a smurf. First Founding Solo Mode gives Ultramarines rerolls in a combat equal to DoS+1 on a Tactics roll.

Wow....that ability is....really good. So if I'm reading it right, any rank 3 or higher ultrasmurf always has all Tactics skills as basic skills, and can attempt that tactics test, correct?

Can attempt the test at a TN of half their Intelligence score, yeah. This is aside from the fact that they can buy every Tactics for cheap on their Chapter advance table, but just in case they don't want to.

As usual, smurfs get lots and lots and lots of love.

Well I've had players whose tactical accumen amounted to "stand in middle of open plain and shoot," so for them it was quite useful.

"Your Tactics (Defensive Doctrine) roll tells you that those boulders over there could serve as cover."

"Wow? Really?"

As a player I've used tactics (orbital drop procedure) to land a drop pod full of explosives on top of a carnifex. As a GM I've used tactics to give bonuses to rolls for imperial assets the team has requisitioned. For example using tactics (Air combat) with a strafing run from a Close Air Support asset to give the roll for the thunderbolts a extra degree of success or a bonus to the ballistics skill test. This works well.

As a player I've used tactics (orbital drop procedure) to land a drop pod full of explosives on top of a carnifex. As a GM I've used tactics to give bonuses to rolls for imperial assets the team has requisitioned. For example using tactics (Air combat) with a strafing run from a Close Air Support asset to give the roll for the thunderbolts a extra degree of success or a bonus to the ballistics skill test. This works well.

Good ideas

My gamers use Tactics kind of like prognostication. They roll it and the higher the degrees of success the more they can glean from the intel that they have and plan around it.

In the emporers chosen lots of the abilites and rolls involved with Heroic Legacies are based on tactics skills. If you need to make tactics rolls to use your aquad mode ability equivelents then re-rolls are going to be very valuable.

As a player I've used tactics (orbital drop procedure) to land a drop pod full of explosives on top of a carnifex. As a GM I've used tactics to give bonuses to rolls for imperial assets the team has requisitioned. For example using tactics (Air combat) with a strafing run from a Close Air Support asset to give the roll for the thunderbolts a extra degree of success or a bonus to the ballistics skill test. This works well.

These are great ideas. I will use Armored Tactics to allow a player a +10 to hit, or extra penetration to synthesize knowing weakpoints on the enemy armor, etc. Right now in one of the games I am in, my character is going to use an Armored tactics test to try and reposition a IG Hellhound to buy more time in holding off an Ork horde.

You just gotta get creative.