You won't have exactly the same reasoning for every sect. For Judaism, it's rooted in a few statements in the Torah, such as Lev. 19:27 and 21:5, where people are instructed not to shave their heads, trim the sides of their hair or their beards. In Orthodox Christianity, the reason isn't primarily scriptural, but beards are closely associated with asceticism and the renunciation of worldly cares. It's customary for Orthodox monastics to not shave, and so Orthodox bishops will typically have a beard, since they're commonly drawn from monastic circles.