It gives you extra grip, if you watch as a night progresses how slippery the canvas gets, it would be a huge disadvantage to go bare footed. Shoes also give you more ankle support, because of the grip you can change directions faster and something UFC fighters don't do as much is that you can drive off your planted foot a lot more with shoes on. This leads to more power in your punches. These are just a few of the reasons people wear shoes in boxing.