In this case: fashion! There are rules for clothes and what one can wear in public, and I find many of those rules to be very strange.
Ok, I get the basics. There are certain body parts that must be covered at all times in public, unless one is in a location where the rules have been very explicitly changed. It is considered important to cover more than those certain parts in most situations so as to avoid embarrassment or faux pas. I am ok with all of those things.
Except then there are rules about exactly how one is supposed to do that covering. Once when I was young, I had a full slip that was designed to go under a dress. To my eyes, it looked pretty much like a dress in and of itself. It covered everything that was supposed to be covered, it was opaque, so it seemed to me to be perfectly adequate covering. So one day I went outside in it, and was chatting with a neighbor. As soon as my mom saw me she rushed outside and brought me in, and scolded me for going outside in my underwear. I got the message that what I did was incorrect, but I have never quite understood why.
Years ago when I worked in retail, one of my co-workers was scandalized by some dresses we once got, because they had adjustable straps. That’s when I learned that adjustable straps go on underwear, and some people are upset at the idea of them on anything meant to show. Ok, that helps explain why the slip was underwear instead of a dress, but I still don’t get why adjustable straps are so awful.
I also find the difference between undergarments and bathing suits to be a little perplexing. They cover the same things, and while bras and panties are not always made to be opaque, they certainly can be. Yet at a swimming pool, it is ok to go around in a swimsuit, but not ok to go around in underwear.
When I worked retail, I was also perplexed at how people treated trying on swimsuits. Apparently an outfit that one would happily wander around in at a pool causes significant embarrassment to be seen in at a store. Rather predictably, I was very confused.
I can’t say I understand these things all that much more than I did when I was a child. They seem arbitrary and senseless to me. Still, I am aware they exist. So I follow the rules, because I have learned that even if I think the game is silly, it is important to play it anyway in order to avoid social backlash.