I have 38C and I wear a bra for a variety of reasons. It does help with support, they weigh a lot and having them bounce freely hurts. It’s like having two large water balloons taped to your chest. Every slight bump or jolt pulls on the skin.

It also helps with sweating. Underwood sweat is a real thing and just as uncomfortable as armpit sweat or thigh rub sweat. A bra helps to absorb sweat and moisture under there.

A bra also acts a a sort of “armor.” My breast’s are rather sensitive, and rubbing against my shirt or other stuff can be painful. Having a bra layer protects them from chaffing and rubbing.

There’s also a bit of vanity to it, depending on type and style of bra. Some are utilitarian and functional only, some are stylish, and some are designed to show off in low cut blouses.

Personally, most of my bras are entirely function and utility as I work as a welder. I’m covered head to toe in safety gear, so wearing a pretty or revealing bra just isn’t practical.

Large breasts are heavy. Redistributing the weight helps the back.

Also for some people seeing nipples is scary for some reason.

Edit: context: a big part of it is weight support.

I’ve got a childhood friend who got a reduction because she had back problems after she went through puberty, and it basically led to her having chronic pain for a while in college. She’s WAY happier at a C-cup than whatever size she had before.

Edit: curious what the downvotes are for. This is absolutely a real thing. Downvoting me for sharing a thing that real people actually struggle with isn’t going to make them struggle with it less. And on the hunch that the downvotes are dudes: if you have a female partner, or a female friend who you can ask these sorts of things to, ask them to confirm that it is real, and they will be happy to.