Huh? In most cases--that I've heard of, anyway--the schools get no money from uniform requirements. (The uniforms are bought from 3rd party vendors.)
The stated advantages are in terms of discipline, since distractingly sexy clothing, inappropriately 'messaged' clothing, gang-related clothing and so on are all ruled out from the start, without being singled out. (One way to think of this is that it's simpler to say what *can* be worn than to say what *can't.*) It also eliminates fashion one-upmanship and the cat-and-mouse brinksmanship games by kids who want to challenge via what they wear.
I'm kind of agnostic myself on whether school uniforms are good on balance or not, but these are the sorts of reasons that I've heard given, and it seems to me that they could be valid.