Beard (n.) A hairy covering of the lower part of the adult male face that reduces the noise-to-signal ratio of the facial area. The result is to make hairy adult males look more like other hairy adult males.
See baldness for a discussion of the interest in asking "why" when studying the human animal. Like baldness, the male beard does not appear to exist for the benefit of women, who are often ambivalent or even hostile to beards. Women do not grow beards. Thus, beards are related to the male environment. The main effect of a beard is to disguise the face. But a facial disguise that takes time to grow cannot work on people who know us anyhow, thus it is intended for strangers. Ironically, for strangers who cannot recognize us anyhow. What does make sense is that a group of men, all bearded, will in fact look similar. A beard is thus most probably a form of uniform, designed to make a man look like his fellow bearded men. Once more, we ask "why?" and the only answer that emerges is the prevalence of male-on-male violence in human societies, and the genetic imperative to, if one must engage in life-threatening fights, reduce the odds of personal risk by looking as much like one's brothers in arms as possible. The variation in bearding in different human groups may be due to different levels of violence, or more likely, due to founder effects that exaggerate original small differences between individuals. The simplest alternative to beards, used in human groups where bearding is negligible, is "war paint".
A corollary to this thesis is that wearing a beard is a sign, not of a violent temperament, but of a peaceful one. The beard is a statement: "I seek no confrontation". It was the Romans who expressed bare-faced violence, while the barbarians were probably trying to make love, not war.