Other than because of things people believe in which have no independent, material existence uniforms serve absolutely no purpose. Why waste parents or staffs energy and the parents or the schools budget on having to get uniforms? That's not efficient and serves no rational, material purpose.
People should base identities in what they do, not what they wear. I know people often do base identities in what they wear but rather than seeing that particular reality as something we wish to maintain we should wish to erase it. It is not a logical way to form identity and leads people into behavior patterns that waste time. People who think clothes are important to the identity of who they are may turn down a good job offer just because of what the employer wants his employees to wear. Granted I think dress codes in the office are also illogical, but a logical person looking for a job will put up with them in order to get ahead.
More and more logic and reason will be important for the human race, particularly in the next coming centuries where we may well be on our way to the stars. Doing-based identities must be promoted over symbol-based identities.
Granted students will still identify with this or that style, this or that clique. But you can't just tell people to be logical and poof they realize why they should be. High school is time to experiment with a wide variety of styles and conformity and nonconformity until you realize that it's all pretty much meaningless and to focus your identity and aims on real, tangible goals, on doings instead of on silly symbols.
When a group of people is wearing the same uniform they are easily identified and recognized by the community. A sense of belonging and unity occur because of uniforms. Likely, students have identity of being a student because of wearing a uniform, thus help them feel part of the group.
Aside from the identity that uniform gives, it also provides security for student. This kind of security is where strangers are not allowed to enter in a school without valid ID and Uniform, therefore, prevents any possible scenario of attack against any of the students.
Teachers and staff can quickly see who should and should not be in the grounds which improves safety and security. Uniforms give kids and sense of belonging and contribute to school tone and school pride. Uniforms help the general public establish whoch students are from which schools and help the school to establish a positive reputation. Uniforms limit the kids judging each other by what they wear and how much money they have.