Because I want to "force" my website visitors to access the forum. If I the public forum, most will not have the habit of creating an account and accessing it, and then it will die. Leaving it completely private only for registered users I make 90% of my visitors create an account and access the forum more often to be able to download exclusive content (rom hacks, games, wallpapers, etc.)
But it is very annoying that it was necessary to "approve" the members who register, because many are complaining that the request was not fast accepted, that it takes so long. This feature of approving members by email sometimes fails and my users are waiting to finally be able to access my site's forum. If you put a resource that releases the entire automatic forum as soon as the person signs up, it would be much better and more practical! Please, do it...