1. It lets me target a banner ad to the top and/or bottom of each forum room. This is done by the call for the ad having the forum number in it.
2. If there is no client ad specified for the position, it will put a goog ad in there - same goes with excess spots the goog ad can be put into rotation.
3. I can sell %'s of each room. For instance, I break a popular forum into 6 sponsors spots, and then have the ads rotate 1/6th of all impressions in that particular forum. Banners can be "weighted" to run heavier or lighters.