Some people want to mitigate your injuries in an accident. Helmets definitely stop you banging your head in an accident.

Other people feel that every accident is one too many, and want to prevent accidents outright. If you hardly have any accidents in the first place, the role of helmets is more limited.

(You should still wear a helmet if you're riding a road bike or a mountain bike or what have you, since those kinds of bikes are more dangerous. You should also consider at least wearing a helmet and taking extreme care if your local infrastructure is not (yet) designed to be safe. )