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.