Can’t speak for all, but back pain and comfort. Also, large breasts can have difficulty getting air under there which can cause irritations. Sometimes it’s just to keep your breasts off your stomach. I wear sports bras and have “special” bras, but my hatred of bras has really dimished when I accepted sports bra life.
I also just don’t like them “loose?” Like, I don’t want to bend over and have them go awol, even if I’m at home. Also, can store stuff in the bra and I have dropped my phone on the floor thinking I had a bra to catch it 😭