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human
ELI5: Why do girls, with virtually no boobs, wear bras?

Hi!

Just thought if this as I, in my hallway, looked at my magnificent semi-fatguy-goes-bodybuilding-body; I've got bigger boobs than my ex-gf and my current. 
Both of them uses bras, but I don't. I don't see why I should wear one. They, on the other hand, who are as good as completely flat with the bras off, uses them.
There's nothing to support, really, so that one reason probably goes away. 

So, why is this? Marketing? Habit? Because girls are supposed to wear them? Trying to make the boobs look bigger like some Spinal Tap-stuffing-thingie? 

I don't want to ask my gf, because I don't want her to think that I don't like the way she looks :)

TL;DR Why do girls without boobs wear bras when there are nothing to really lift/support/whatever?
gpt
Firstly, there is often more breast tissue than is immediately apparent. For wide-rooted breasts, a significant portion of breast tissue can lie around the underarm area. So don't underestimate the mass of boob just because they don't stick out much at the front! 

A well-fitted bra will support all the tissue and cup it so that the stuff which tends to lie at the sides is comfortably moved to create fuller breasts and more cleavage. 

Breast tissue can also be very uncomfortable when moving quickly, especially when jogging. Imagine wearing a hoodie, with something heavy in the pocket on the front. You know the way it bounces around when you run? That's how boobs feel, even small ones, and it can be painful! The right bra eliminates that. 

They can also protect sore breasts and nipples from rubbing, smooth outlines under clothing, and make you feel good about your body.