I know two people who had brain damage (like, serious damage that persists years later) from low speed falls - one was literally standing still, and the fall sent them to the hospital for a week. The other one was low speed, just starting off after stopping at the bottom of an uphill section. Not sure the exact details of what happened, but same story - fall -> hospital -> permanent loss of cognitive abilities and some change to personality.

Totally agree that if a truck runs you over, helmet probably won't do much. But a helmet will totally be the difference between an embarassing fall with a few minor scrapes, and one of the most (negatively) impactful moments of your life.