For me, the biggest draw was competition. I enjoy going to the track, but for different reasons. Overall, lapping days don't offer me the same thrill of competition that sanctioned (SCCA) autocrossing does.
I think autocross also makes you a much better track driver. When you only get three runs, you learn very quickly how to read a course or track. In my expereince, students with some autocrossing experience make much better drivers on the track. They pick up the line quicker and tend to have much better situational awareness and car control abilities.