Exfoliation encourages removal of "old" skin cells and ensures that the old cells don't get trapped inside the pore. For oily skin it means less blemishes because you are helping to prevent the pore from becoming blocked. For dry skin it means you are encouraging removal of dried up skin cells and so that new hydrated ones can come to the surface.
When performed correctly, exfoliation helps renew the surface of the skin, leaving it brighter and more radiant. It can also allow other products, such as serums to penetrate the skin's surface more effectively
Exfoliation is a necessary part of the skin care regime and should be done twice per week (but more if the skin is excessively oily or dry).