According to one theory, there was a time when a pimply mess on your face kept the opposite sex away while you went through the process of becoming a finely-tuned reproductive machine. Adolescence produces an interest in sex, but it doesn't magically provide you with the experience and maturity required to raise a child (or to defend it from wolves, if we're thinking in terms of evolution here).
So acne provided a buffer period, giving sexually-motivated adolescents time to learn how to be responsible. Because while bad parenting today merely means a visit from the government and/or a reality show contract, historically it meant dead children, which was a huge waste of resources -- and also prevented people from passing on their genes. Destroying the topography of a teen's face may have been nature's way of buying them time to realize they wouldn't make it as a wandering cave artist, get their hunting and gathering act together, and give their eventual offspring a better chance of survival.
Nature could have simply delayed sexual maturity, but nature likes watching you masturbate.
Another theory suggests that acne also made younger males look like less of a sexual threat to older men in the community, which protected them from harm until they were old enough to protect themselves. An older guy would look at some pimply-faced kid and conclude that the dweeb had no chance of stealing away his woman, and thus wasn't worth the hassle of bullying. Theorists are still working on how to make this theory jibe with everything we know about high school bullies.