Does it do anything other than making them available for pick up in a level? As in, is there any way to equip them before hand? Also, what's with the picking up of items from "your friend" in between missions? Any point to it? If it matters, I'm playing on vita.

@mars: You're correct, "unlocking" weapons merely allows them to spawn in a level. The only way you can start with a weapon in hand is by using certain masks.

The significance of "your friend" that you encounter between missions is somewhat open to interpretation (I assume you're referring to the guy at the pizza place/video store/convenience store). I urge you to finish the game first, then look up some of the threads on here (or the Steam forums, or wherever) that go in depth on the plot and how all the characters fit together.