It varies from person to person. If you see a mustache or a goatee, it's probably just a matter of style--the person likes how it looks. Others grow winter beards to help keep a little warmer (it's not really effective) in the winter time.

Personally, I'm clean-shaven, because I've been working in fast food for four years, and it's *required*. This weekend is my last weekend in fast food, though, so I'm going to grow a winter beard for sure this winter. Beards look like hell on me, but I don't care.

Actually, I was considering growing a beard starting at the end of my last shift at Wendy's, just because I can now. I dunno. We'll see how things go.