When we originally started there were more mods than members logged on, o it looked kind of "cliquey".
I dont know as we do still need it, just one of those things I have never got round to changing.
One of the other reasons I set mine to invisible, was that I used to leave my session logged on whilst running backups etc whilst I was not at the PC.
People could have sent me PM's asking for discount codes etc, but may not have received a response, as I actually wasn't there. So logging in as invisible removed that problem.