As the Magician said, it's SOP in the UK. It's How Things Are Done. I've avoided being on the committee (in part because no sane person who knows me would *want* me on a con committee), but I spent a fairly large chunk of most Redemption cons running around Doing Stuff as the conchair's personal gopher. I got a free mug one year, and that's been it. You do it because it's a way of putting back in. It can also be useful for one's CV, but I don't actually know anyone who does it for that reason. (I think.)