Why do Cats scratch themselves while they're grooming themselves?

I get very scared when they do this whether they're doing this while cleaning themselves or anytime my current cat scratches himself and I get really worried because I had one cat previously scratch himself an infected abscess, and incurred a very expensive vet bill. So I really want to avoid this again. So if I see it I hurry up and scratch for him so he doesn't do it with his claws. Should I be worried if he scratches himself at all?