In 1982 I bought a Bell bicycle helmet and rode with it (and several replacements) for years without getting into a situation where I needed to rely on it for protection. Then in 2007 a dog came after me and ran under my front wheel. I hit the dog, then the pavement. My helmet did its job sustaining a large crack as it protected my head. I had a slight headache, that's all. After 25 years my helmet wearing paid off.
I did not realize how fragile our heads are until a few years ago when a family member tripped, fell and hit her head on the street. Due to the resulting migraines, memory loss and other cognitive difficulties she was unable to continue her work as a neonatal RN and had to retire early. Now I am even more vigilant about wearing my helmet.
A cynical fellow rider says that another name for non-helmet wearing riders is "organ donor.'