A lot of games are starting to allow this. Say you come across an armor item that is better then the one you currently have, but you prefer the look of the old one? Well, you wear the new one, but you put the old one in it's costume slot, lets say for this it's a chest piece so you put it in the chest costume slot. You have the look of the one you like, but the stats of the one you need. This could pave way for costume only items as well.