The need for support varies a LOT with size. I never wear a bra in my own home, and I rip the underwires out of the ones I wear outside. My breasts sag, but that’s normal at 60+. But I’m a C cup, so they’re only protruding about 3 inches even when supported. And I only do low-impact workouts for other reasons. I wear bras to put my boobs in the parts of my clothes that are designed to have boobs in them, rather than falling out the armholes or something.
A large-breasted woman with three or four times the pounds of flesh hanging off her ribcage is not going to be comfortable slinging them around braless all day, especially when they get out of sync, as boobs are wont to do when you’re moving around. Especially if she’s running or jumping. Her bra has to work hard, and requires more engineering than mine. She and I have the same underlying muscles, tendons and bones, but her whole balance is under much more stress.
Meanwhile for an A cup woman a bra is probably mostly for fashion. She could have completely flabby musculature underneath and they’d still be perky.
If men had balls the size of boobs they would understand better how they can get in the way. And go 2 different ways! And it would probably be difficult to design comfortable underwear for them.
Bras were invented by women, for themselves, for their own comfort. Yes, SOME bras are unnatural and fashion-driven, and even those serve an occasional purpose. If you’re a woman who doesn’t ever wear a bra, great! Enjoy! Nobody should ever force anyone to wear a bra against their will. But if you choose to go braless, remember it’s not necessary to disparage the women who make a different choice for their own reasons.