All right, kids. I grew my first beard when I was 18. It was full and thick and auburn, and the sidewalks sizzled under my feet. For several years, I'd shave it off in the spring and grow it back in the fall. I met Dave, Dave likes a full beard. I no longer shave, but I maintain other areas, and I keep the beard shorter than homeless length.
Recent studies suggest that people are attracted to the more exotic-looking man, the unique man. When most are clean-shaven, the beard is more attractive, and vice versa. These guys are looking to be different, just like everybody else, and they lack individuality or imagination.
As for the rabbi look, the Amish beard with the bare upper lip, or Duck Dynasty latter-day hippie, if it looks good, wear it. Don't use a beard to ugly yourself up.
As for why, that is usually the most useless question there is.