Why do women wear them?
The only real reason I do is to keep the world from seeing my nipples through a light colored shirt, but I’m pretty small. If you’re really well-endowed it’s a LOT more comfortable if they aren’t dangling in the breeze. Gravity will get you eventually regardless of whether you wear one or not, but I think for most it’s a matter of comfort.
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While somewhat uncomfortable themselves, they are a lesser evil than having two pendulums swinging loose all day.
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They make breasts look better by allowing them to maintain their round shape and fullness
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Not wearing one has been associated with a certain moral turpitude.
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They help maintain the shape of the breasts in the long term by supporting them against the stretch of gravity. Ever seen a woman in National Geographic whose breasts warrant entrance into the Teet Hall of Fame?
I’ve seen a lot of “Well endowed” women that don’t know what they are for (apparenly) 
Seriously, men are similar. I hate boxers as my “endowment” which I must admit is average, goes where ever in boxer but is OK in briefs.
Yeah, chicks who never wear bras have nastyass boobs. They can almost touch the ground. Ewwww.
Bras keep your breasts from bouncing about uncomfortably when you do physically active things (like walk). Mine are relatively small so I don’t need one for simple walking, but if I have to run, or even walk fast, without one, it feels like my nipples are being pulled off.
The German term for the brassiere is “stoppemfrumfloppen” That should explain it.
Breast tissue can be pretty tender and as Kelly pointed out, the slight bouncing from walking can actually be uncomfortable. The support from a bra helps to mitigate (or eliminate) that discomfort.
When I do martial arts, the sport bra I use has so much support, they don’t move!
Also, for many women, long hours without a bra is downright painful, particularly for the back. Many women have very dense, and therefore heavy, breast tissue, which puts considerable strain on their back when they are sitting up or standing or walking around all day. My best friend actually finds it hard to go more than a couple of hours without a bra, and will often even wear a sports bra to bed if her back is hurting her excessively. She plans on having a breast reduction done eventually (likely after she has had a child or two) to help alleviate her pain.
Myself, it’s a combination of the less-nipply reasons, the basic comfort of not having them wobbling around so much, and a bit about the pain. I have a bad back to begin with, and after half a day or so without a bra, I begin to feel the strain a little more than usual.
How does a bra compare to a bodice, then?
Both accomplish pretty much the same thing: keeping your boobs where you put them. As far as comfort, I much prefer a bra because each breast has it’s own place rather than just being crammed together (TMI? sorry) and mostly because with a bra you’re being squeezed all around by about one to two inches of stretchy fabric rather than a foot or more of an I-beam support structure. You should see all of the Ren Faire chicks at the end of the day. You can almost hear the synchronized pop and sigh as all of the bodices are let loose. Ahhhh…
Hope this is enough, but not too much, information to answer your question.
If not, I’m sure at least one guy out there will derive something from it.
I prefer the man-ssiere.
Another reason, tied in with KidCharlemagne’s second reason, is for attraction. Two well-placed orbs are more aesthetically pleasing than asymmetrical cylinders with nipples pointing askew (I know; there are bra-less wonders out there, but you gotta admit they’re in the minority).
I think you mean the bro… 
On a particularly bumpy train ride, the frequency of the train car motion hit the resinance frequency of the girl’s boobs sitting across from me. She was sort of embarased and I could only suppress busting out laughing by looking at the painful expression she was trying to hide. After quite a few bounces which seemed like they would tear them off, she finally crossed her arms under her boobs to stabilize them. She looked at me and gave an expression as to say, ‘yea it sucks, what can you do?’
It was eyecatching but I felt sorry for her as it was obviously painful, I could see how a very supportive bra could have helped. She added at least a 1/2 inch sag that day.
Heh. Yeah, nobody wants to get a fat lip while jogging.
According to Better Midler, they were invented by Otto Von Titzlinger.
Hmm… I had no idea bras were so useful. I’m guessing they aren’t a recent invention.
However, times change and this seems to go in cycles. When my mom was filling out, ca. 1920, it was considered morally scandalous in some circles to wear a bra; a symbol of depravity not unlike the reverse concept of the bra-burning 1970’s.
Arf arf!
“According to Better Midler, they were invented by Otto Von Titzlinger.” (tit-slinger ?)
Nope, far as I know, they were invented by a woman.
they are for covering & supporting the breast.