This is a comment from a players perspective, not a developers perspective.
If you play a wide genera of games, you will likely have encountered a situation where your avatar makes zero sense for the game you are playing. Instead of having to to though the avatar customization menu every time you want to play it and having to change it again when you play a different game, it would be nice if you could set that game to use a certain costume.
This could also have a side effect of driving more avatar item sales. I don’t have any data on this, but I am guessing that most players don’t change their avatar all that often, and as such, when buying avatar items do so with a mindset of “this is something I am going to be wearing all the time”. With per game customization, I would guess players would be more willing to buy items with the sole purpose of using them for a handful of games.