The purpose of the uniform is to help make the organisation or company look more professional. That's why uniforms are there, regardless of whether you are a pilot, policeman, nurse or indeed a pilot in training. Do you not think that OAT, OAA, FTE, CTC know the airlines are looking for the professional looking, clean cut cadet?
I suppose the other purpose (less obvious/measurable) would be it gets the students in a 'working' mindset where they feel like they are doing a job. Suppose it gives you pride in what you're doing (cynics, you'll have a field day with this one
).