In North America, more than 90% of women wear a bra; interestingly, though, most wear it for "cultural"
reasons, not for the breast support (2). Here are some reasons why women wear bras together with some counter arguments:
1- Support: This is considered a myth among many experts who performed studies in the field (check resource 2). Probably in the case of heavy weighted or large breasted women the case might still hold true to avoid slapping and shaking while moving and the sweat underneath. Otherwise, it is a personal choice related to comfort and not actual support.
2- To avoid sagging: Bras do not prevent gravity's effect and your breasts will eventually sag and fall a bit down--nature. In fact, a study suggests bras might increase sagging of breasts (Professor Jean-Denis Rouillon).
3- To cover the nipples: This might be a legitimate, expected reason for many women, but if you don't want to wear a bra, you don't have to only for this reason. Some alternatives include wearing camisoles/vets that give decent shape to breast and partly cover poking nipples, wearing breast cups, or wearing nipple covers. And in all cases, society (which is all of us) should know better than look down at such a natural thing as nipples if they poked through during normal day activities.
4- To avoid their bouncing during exercise: Try wearing sports bras. They are much more comfortable. Bouncing of the breasts can be harmful to the ligaments inside the breasts but a small movement is probably useful for the lymph flow.
'It should not leave marks on your shoulders or under your breasts, or feel tight.
Be especially careful about the underwires and side panels, because if the fit is not just right, the underwire can poke into the breast tissue, and the side panels can create extra pressure and tightness.'
A bad fitted bra "can cause back problems, muscle strain and poor self-esteem" (3). Check resource 3 for more on fitting your bra! But remember sometimes even if you get professionally fitted, size may change from brand or style to another just like shoes!
Use the help of a professional fitter to find your fit and right size.
Health before fashion ;)
Finally, here is a video on bra fitting:
1- Brassiere: http://en.wikipedia.org/wiki/Brassiere
2- 007 Breasts: http://www.007b.com/why_wear_bras.php
3- Bra sizing: http://www.lovelyish.xanga.com/773045292/bra-sizing-does-this-fit/