Belonging to a specific group is, in itself, a definition of being Japanese.
Wearing a common uniform is one way of conforming to a group identity.
Employees at banks wear uniforms, street workers wear the same yellow jumpsuit, and it could be said that homemakers wear a uniform because they wear aprons, even out and about shopping. There are school uniforms (including shoes, backpacks, and hats) for classroom and physical education. Preschool uniforms are artist smock-like jumpers worn over other clothes.
For more about the importance of groups in Japan, click here.