Skin or attachments will give the server a chance to keep the weapon count in general low ,there for you will be able use less guns and all will have different stats ,as a player would be able to buy a gun from a random gun shop in the basic stats but when you craft the stats and skin will change, would be able craft skins,attachments,maybe new abilities(for melees).
As much as you craft your weapon as rareress it becomes and gets expensive ,(providing money for the server)