So on multiple occasions, I have heard people on this forum speaking of using side arms/ weapons in their off hand. However, I have never seen the value of this, outside of maybe the DDC Defender and Greedo's gun. I would assume that this concept is giving a pistol to a mid-long range fighter or vice-versa. However, in most cases, I have noticed that most on-hand weapons, once modified, are going to be so much better for the hero, that there would be no need for a secondary weapon. Is there something I'm missing? Is there some sort of reload mechanic I'm missing that only allows you to attack with one weapon each activation, even though you can perform multiple attacks?

A hero can take 2 weapons into a mission. When declaring target, the hero also declares which weapon is used for the attack.

If you don't need to sell a weapon to get better items, then it's good to give a ranged weapon to a melee character. It can sometimes be used to recover strain while finishing off a figure in a deployment group to reduce imperial activations and prevent reinforcing the group.

Applications of having two weapons that I can think of quickly:

Gunslinger

DDC defender

Keep a spare weapon with stun

Melee and ranged on Verena

Force pike and a weapon with cleave on a melee hero

Ranged option for a melee character

"I don't know when I'd use this but I don't need the credits I'd get from selling it"

Short and long range options

shun-Yen's Lightsaber, Greedo's Pistol, Stun Baton or other weapons that have an ability that doesn't attack.

Either of the focus guns from Twin Shadows so you can kill something and focus when it's on sufficiently low health.

I did not know a hero could carry two weapons. So they declare which weapon they are using? Is it specifically designed "side arm"?

Any two weapon item cards (but starting weapons can't be traded to a different hero) with their appropriate attachments. (See Attacks from the RRG.)

You're basically never going to have the extra credits to do it unless it's to help Jynn. However you're better off bringing a pistol along you haven't sold yet than just having credits in the bank because a rare situation might come up where you need to do a ranged attack with a melee charactr. However you are almost always better off just doing a rest action or opening a crate than taking a shot that might not have the accuracy or might get blocked and do no damage.