What are the advantages of getting a breast prosthesis?

Psychological. If a woman is not ready or not interested in breast reconstruction an external prosthesis is often a simple solution to wardrobe problems and social situations. Modern breast reconstruction should always be thoroughly discussed as an option.
Brest prosthesis. Assuming this is post mastectomy: a well fitted prosthesis will make your clothes fit better and the chest wall appear symmetrical to others. Ti's for comfort and psychological benefit.
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.