See
this post under "Useful Information". When the system is running, you can use your credits to purchase (among other things) x10Hosting service upgrades, domain names, and so forth. It takes a
lot of credits to buy things, though -- and the credit system was down altogether for a big chunk of my time here (I had more posts than credits for quite a while). There's some custom code built around vBulletin for the system, and every upgrade or critical patch knocks the custom stuff for a loop.