Let me give my opinion on this since... I bought a pair.
Breasts are a symbol of femininity, just as muscles are a symbol of masculinity. Breasts are part of the image of what a woman should be, along with the hair, legs and hips. Its the image of a vixen, a sexually charged woman. A woman without breasts can sometimes feel like a man without muscle.
Not only that but breasts do women the favor. Larger breasts and being top heavy makes your stomach, arms, and legs look thinner. They add the curves and fill out tops and when pushed together its a deadly thing. ;]
The downside is that some clothing do not support breasts and high end designers do not make dresses for women with anything bigger than a B cup. Breasts are hated in the high fashion world. Large breasts also take away from if you have longer legs and make them look shorter. Also at times, breasts make you look chubby. Breasts are fat, of course, but you get the double boob armpit affect if you have the wrong outfit! And just as males get stretch marks on their shoulders and arms, women sometimes get them on their breasts.
Just some stuff I've thought about! =]