To be able to give Uke some colours and textures to make him a little less bland. You could also use it to see how your textures look in the game and see how it works when moving and kicking and punching etc. Also make your replays look cooler.

Take other people's textures from custom folders. that is all. It's illegal to put them on you though. I used Oblivion's textures on my uke.