In the beginning... No, wait.

It was a dark and stormy night... That's no good, either.

Fine, a straight answer. In network operating systems, there are often so many users it is too tedious or time-consuming to assign system privileges to each user. It's much simpler to assign a group profile the necessary privileges one time, then assign each user to a group. Hence, we have registered users, contributing members, moderators, and so on.

Why this is remotely interesting is a mystery to me, but there's your answer.

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"I laugh with humanity, not at it, because I am as big a fool as anyone, and bigger than most." - Dean Koontz