Firstly, in the rare event of an emergency (such as a natural disaster) identification of students by their parents could be difficult if all children are wearing the same exact things.
Secondly, uniforms can erase the expression of a child's unique individuality. It could be a bad thing to take individuality away from people. School's primary function is to educate, but it's also a platform for socialization where clothing can play a role. Furthermore, how you dress and the way you act make a huge impression on people.