Simply put, it's to help pay for the cost of the hosting/server space.
As we grow, the out of pocket costs for hosting increases. This will help defer some of the costs. We will not directly ask you to click through to the ads (hint hint), but if you see something that interests you, feel free to check it out. Just wanted to let you know why they are there.
Rest assured that the ad click revenues support ongoing maintenance and improvements to our forum. We're talking pennies here, but it adds up.
Thanks-
Scott and Chris