I am not sure what he is doing, but one possible use for this type of configuration is to allow multiple websites to share the same forum, yet have their own look and feel. That way, when people post in one website, their post appears on the other, and vice versa. As a result, the forum builds more users and more posts than it would otherwise. And more users and more posts attracts more users and more posts. It speeds growth.
Actually, I have seen this done before with a forum, and it worked quite well for them. Within a year, their forum has become quite busy. They currently have about a dozen websites who have their own skinned version of the forums all sharing the same database.
I liked the idea so much, that I am going to do the same with one of my forums. We are already putting together a package that will allow websites that complement ours to have our forums on their website. This arrangement is great for smaller sites who want the stickiness that a forum creates, yet does not have enough traffic to generate enough posts to sustain an active forum, yet has enough traffic to bring new members and posts.
Plus, as an added bonus, our website, which we are developing into a portal, will get recognition and advertising on other websites. It's a win-win situation.
(Wow! This is my 100th post!) 
Edited by wistex