why are horseback riding trainers picky about clothes?

i am riding at a new barn and today was my first lesson. when i got into the ring the first thing she said to me was, you need to come in a polo tucked into my riding tights with a belt…

why does she care?
does it make a difference in my riding?
OR does it just look better???

Yes, it DOES make a difference.

You need to wear a tighter shirt (like a polo) so that she can see your back and tell it’s straight. If you’re wearing a baggy shirt, she won’t be able to tell if you’re hunched over or if you’re sitting up straight.

You can ride perfectly fine in baggy clothes, but nobody can tell what your position is like if you’re wearing a loose shirt or something. So your trainer wants you in tighter clothes for your lesson so she can better see and evaluate your position.

She wants you to wear a belt so the shirt doesn’t come out of the tights and start flapping around.

The tights are because she wants you to wear tallboots. They improve your leg and are easier to ride in than paddock boots.