The benefit is in letting players display information the way they want it displayed. For instance, maybe I'd like my hotbars to be a shape other than straight lines. Perhaps a 3x4 box would be more useful to me. Perhaps I'd like a hotbar that only displayed 3 buttons instead of all 12. Perhaps I'd like that hotbar to be click-through. These are all highly specific preferences and not worth adding to the base UI. That's where addons come in.