You get extra features. Someone on that site that I use to play with fro time to time donated money a few times and one of the times told them that he was doing it for me, so I got all the features for Reach.
Well, I'd consider giving them 5 bucks, but 15? No way. It's an okay site, but I don't care much for the community. I've been posting there semi-regularly for a year or so, but I've probably only found a couple people that I enjoy playing with. HaloCharts was so much better.