Maintain balance. Most people aren't aware of the weight in a breast until its gone. A proper fitting prosthesis replaces this in addition to providing a proper clothing fit. Subtle shifts in weight/balance can lead to neck and shoulder problems from the body's attempt at compensation. The larger the breast the more significant an issue this can be.
Breast Prosthesis. A prosthesis allows you to look relatively normal in clothes and lets you avoid any additional surgery.