Occupation seems to be a standard question when you register with a new dental practice. Why? Surely the treatment's the same. Or are they trying to weed out potential clients who might get their teeth knocked out during the course of their work (like boxers and... errr...).
Presumably there's an obvious answer I'm overlooking but at the moment the only thing I can think of is a recollection that agents dropped into occupied Europe during the war had their fillings redone to match the continental style so they couldn't be ID'd as British if picked up by the Gestapo.