Australia

Conformity, the theory is that kids will behave better when they all look like each other and can't single each other out. Which is completely wrong, as we'll always find ways to separate ourselves from each other.

Schools are full of bad administrators who are poor at managing those they're responsible for, and a hallmark of bad management is blanket policies, dress codes are an example of this, and dress codes begin a slippery slope of what a student can or can't do, including hairstyles, extracurricular activities, what they read, and more.

I always thought it was funny that bringing in blazers was the sign of a failing state school, in the UK at least.
Ofsted good/excellent? Polos and fleeces are fine, it's all good.
Requires Improvement/Inadequate? Shit, better get all the scrotes in cheap blazers that cost £50 so we can ape the private school down the road.

The flip side of the coin is, if parents buy cheaper uniforms they don’t have to buy more expensive, name brand clothes for their children. The school also avoids situations where kids with wealthy parents bully kids from poorer families.

There is probably also an argument for it helping to build school or team spirit, unity etc etc

School uniform for me back in the day was a white shirt, grey pants, and leather shoes. The only "school" part of the uniform was a tie and blazer with the school crest on it. And I even went through all of high school never purchasing said blazer.

This was in NZ nearly 15 years ago.

My point being, uniforms shouldn't be breaking the bank. If schools would simplify the uniform as to not be that unique to the school, they might be able to drive down cost as more readily available clothing could be used to make up the uniform.

Because public schools have to compete with private schools, the uniforms make them look comparable and has more of an effect on the parents perception of value of the school.

Each public school gets funding dependant on enrolment, the end result is absurd pricing for single income parents. When I was in High School we could get a $7 shirt from Big W and look identical to other students minus the logo.

There are exactly two reasons why schools mandate uniforms: greed and/or authoritarian leanings.