Birth control pills were primarily brought on the market to prevent unwanted pregnancy. Since then they've been used for other purposes, like regulating menstruation or, in a recent development, stopping the monthly flow entirely.

How about treating acne? Acne flare-ups often start at puberty and become worse during menstruation and pregnancy. American Chronicle gives us a reason why this is so:

Androgens are a group of hormones. Androgens are also called male hormones. These hormones are responsible for activity of the sebaceous gland. When androgens are in excess the glands produce more sebum and as a result more acne. Use of birth control pills and other hormones is done to reduce the activity of androgen s to treat and control acne.

The Blog That Ate Manhattan tells us that this isn't anything new. It's just not publicized:

Although only a few brands have bothered to get the FDA approval to be marketed specifically for acne treatment, we docs have been using pills for this indication for years. Almost any brand of pills will do; I recommend ones containing the progestins norgestimate, norethindrone, drosperinone or desogestrel. I prescribe pills frequently for this condition, and trust me, it really works!