They are compulsory in my country (Australia) and it is very beneficial for the following reasons.
It promotes safety, teachers can quickly see which students should and should not be there.
It helps the school develop a special identity in the community and gives each child a sense of belonging to a unique environment.
It eliminates competition about who wears the best clothes so kids are judged on what they do and not what they wear,
They are sunsafe and appropriate for the weather and activity.
They help students to feel like students which in turn helps them act and learn like students.
I think students should not have uniforms because it doesn't show their true personality, the uniforms are uncomfortable, and it costs less money to where something that you already bought then to got out and buy sets of clothing with money that you save for better and more serious occasion.