I'm in the UK - the first primary school I went to did not have a uniform, the second I went to, and the secondary school I went to, both did. I think there are benefits - although the uniform policy at secondary school was not enforced as strictly as some, so people were able to personalise their uniforms up to a point (for instance, officially girls were all supposed to wear navy blue a-line skirts - in practice, as long as the skirt was navy blue and fairly neat and tidy it would be allowed) It did not prevent some girls from being more fashionable or trendy than others, but it did make the differences much less obvious. The school did have some 'spare' uniforms (mostly things which people had failed to reclaim from lost property, some which were donated by leavers) and this did allow those who didn't have suitable uniform to be helped discreetly. I was completely unaware of this when I was a pupil, I only became aware of it when I was an adult, as my mother worked for the school. No one other than the child concerned would know, and the fact that the uniform was fairly standard meant it was not obvious if someone was wearing a 4 or 5 year old blazer, for instance.
it did 't prevent bullying (as I know to my cost) but it did reduce the ways one could become a target.
At my school, we did not have to wear uniform in the 6th Form (6th form was age 17-18, and was after the end of compulsory school age) but we did have a dress code - as I recall, we were not allowed to wear denim (later they explicitly forbade leather, too!), sleeveless tops or short shorts or skirts.