I believe that's the thing Sho. The rewards is to make sure we don't just inexplicably have things on a whim, or as a sort of more tangible means of applying our credits. While I'm sure none of us wants to try to simulate the life of student poverty, if we're given a tangible amount of money after missions that gives us a rough estimate of what our spending habits should be. Bullet's aren't free, and if Gren wants anything more then Hollowed Points and Armor Piercing, he'd better have the money to regularly supply it himself, and a good explanation of how he got all those toys.