As Bradleigh explained.
One thing to be mentioned: It helps to detect Trolls. If you see somebody post very aggressive or controversial, you can have a look at his "rank" or number of previous posts and in most cases that way you can make out, that he only wants to make trouble and is a new account. People who post on a regular basis are more likely to watch out for their reputation on the board.
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Deep into that darkness peering, long I stood there wondering, fearing,
Doubting, dreaming dreams no mortal ever dared to dream before (E.A.Poe)