The female breasts have very little internal support, being composed largely of adipose tissue. It is believed that the primary anatomical support for the breast is provided by the Cooper's ligaments, with the skin covering the breasts offering some additional support. However, this anatomical support is usually insufficient to hold the breasts up (especially in older women), and to prevent movement of the breasts, which can cause pain and discomfort. The primary reason for wearing a brassiere or similar foundation garment is thus to provide external support for the breasts, both to increase comfort and to provide a sexually alluring figure.

Bras are believed by some to help preserve the youthful shape of breasts (which naturally sag as women grow older), an opinion which is unsurprisingly promoted by bra manufacturers. However, there is some doubt over this amongst the medical community, and while a woman may choose to wear a bra for comfort or for social reasons, there is no conclusive evidence to support the notion that a woman's breasts will sag lower over her lifetime if she doesn't wear a bra.