One might as well ask why professional musicians need a conductor in order to perform together in an orchestra. Aren't they all experienced professionals, don't they know how to read music, can't they just do what needs to be done without someone standing before them waving a silly baton?
It's not that the BOTF [for instance] needs an editor to always be on hand but there are times when it IS necessary for someone from on high to enter the scene and kick a little rump. This is why ALL chatrooms across the Internet have at least one moderator in attendance.
On the BOTF board there was one poster spamming the board with inflammatory top posts in a non stop manner and he did not care if anyone replied to him or not. Good posts were being knocked off the page by his rash of non stop garbage posting. An moderator could have fixed the problem in jig time but with Geoff departure in December [he no longer works for Slate -- he returned just to help out temporarily] these Fray boards do not have a moderator and the nut cases are gradually becoming aware of that fact.
So it goes.