Let me preface this by saying I am a man, and smoked a little too much, so I'm sitting here thinking... what is or was the original purpose of a bra? Weight support? Vanity? Covering the nips so people's eyes met your eyes and you can have a normal conversation? Like what's it all about?
This. At least that's my understanding. When I lived in South Florida, one of my friends said she was going to try going braless for the summer, a few weeks later she complained that going braless made the under part of her breasts chaff / irritate. Heat, humidity, and skin friction is bad mmmmkay.
This is why I feel so so lucky to be a small-breasted woman who does not live somewhere hot 24/7.
If I ever do put on a bra it is because I expect I'll be running (only situation I have been in where breasts hurt braless, very thankful), or because the silhouette of my nipples is visible through my clothes and I don't want it to be. A good deal of my wardrobe won't show nipple even if I go braless, but not all of it is like that and so I put on a bra.