Breast asymmetry can ranged from very minor to gross asymmetry involving volume, shape, nipple position and sometimes, underlying rib cage asymmetry. The aim of surgery is to help correct/improve these asymmetries.
Breast asymmetry is extremely common and probably around 95% of females have some degree of difference between the two breasts. The asymmetry can be related to shapes, nipple height, volume and in some cases, there are issues such as breast constriction, specific areas of tissue deficit, over-projection of nipple areola complex (Herniation).
For most females, this is not an issue but for those with 1 or more cup size difference between the two sides, finding bras that are suitable, going swimming or undressing can become an issue.