Uniforms bring people together. Though not all martial arts require practitioners to wear the same outfit, most do. There are a number of historic and thoroughly modern reasons for this tradition.
Sporting a unified look while learning to fight goes back to when martial arts were used by military groups. Uniforms reminded everyone that they were a team, fostered loyalty to each other and dedication to quality.
This spirit of teamwork is still present in modern day martial arts, but uniforms do more than that. They also give students a clear cut way to change their mindset for practice. Wearing the same thing each practice allows you to associate that outfit with the activity, and thus improves your focus. For some, the simple act of changing into your uniform can get you in the zone and ready to work.