Paula Marantz Cohen:

I have never known my husband without his beard, a fact that disturbed me in the early years of our relationship. What was he hiding: a weak chin, a saber scar, a slothful nature, a psychological need for a barrier between himself and the world?
…My son, at 25, sports something that alternates between a five o’clock shadow and a two-day stubble. I personally am not crazy about this; it looks as though he has forgotten to shave. But I suppose this is the point — the look is an affectation of forgetting to shave, not a real forgetting. As I think more about it, the shadow/stubble carries interesting resonance. Unlike my husband’s clipped boxed beard with its sense of modesty and stability, my son’s quasi-beardedness is more whimsical, more a self-proclaimed mask. It also announces its transitory state unequivocably: No sooner is it achieved than it is erased in order to be begun again. This seems emblematic of our current Internet culture which is ephemeral and continually in need of updating. My son’s quasi-beardedness may also reflect a slowing of the progress from childhood to maturity, what sociologists, referring to the years between 20 and 30, have dubbed “emerging adulthood.”
Ahem. You know, I’m all for indulging in fanciful, quasi-poetic digressions about ordinary topics, but if I may, a prosaic explanation: perhaps, like myself, some men grow beards (to whatever degree) because they like the way they look with them. Maybe they simply have no fucks to give when it comes to fashion and the chattering imbeciles who pay attention to it. Could be that they spend enough time outdoors in cold weather that every little bit of extra insulation helps.
All valid reasons, and ones I personally endorse. I would add that sporting a beard also associates one with a sort of bohemian-poet-musician archetype, which suits me fine, and when paired with my favorite style of hat, the Fidel cap, it adds a bit of revolutionary panache to my proclamations of being a Grouchomarxist.