Let me preface this by saying I am a man, and smoked a little too much, so I’m sitting here thinking… what is or was the original purpose of a bra? Weight support? Vanity? Covering the nips so people’s eyes met your eyes and you can have a normal conversation? Like what’s it all about?
It’s not hard. Put on a really heavy backpack and leave the straps super loose, and go try to move around, maybe a few athletic moves that involve changing speed or direction. Compare to a tight backpack with a waistband and shoulder straps properly strapped to your body, and try to move around again. The straps help control the extra motion so that you’re in better control.
Or run around in shoes 5 sizes too big. Or go for a run with your arms loose and intentionally left limp, swinging around like pendulums.
The whole world has a million examples of why providing bracing and support makes for more efficient and comfortable movement.