My main reasons for asking for it concerned individual interviewing styles, setting up some sort of loosely defined "roster" to make sure we didn't have three or four people requesting the same person give them an interview, posting up question lists to give each other ideas (not carbon copy questions) about things to ask about, little details and assists that don't really belong in an open forum where we'd get 50 replies from various people who aren't involved.
We could do it via emails and IMs, but setting up that kind of stuff takes ages, and it'd be weeks before we got any kind of organization happening. With a private forum we could have a reasonably organized system running with a minimum of fuss within a relatively short period of time, and we'd have an easily accessed source of interviewer only feedback and suggestions to help solve all the little problems and difficulties we're sure to run into during the first few weeks on the job.