I actually rode without a helmet for over 40 years. About a year ago, after one of my friends was in a coma for a month after falling off her horse, my DH and I bought helmets. The first time that I put mine on, I realized that it was a good idea.
I had not been thrown in years. Something startled my horse. She jumped to the side, then took off at a gallop. Years ago I would have had no trouble with that. Now between bad balance and weak legs due to a neurological problem, I just couldn't get back up and I bit the dust. I didn't actually hit my head, but I did think about that I could have.
After I hit the dirt, my horse stopped galloping, came back, and she looked at me. I could almost hear her thinking, "What are you doing down there? Shouldn't we be riding?"
I found a ditch for her to stand in and managed to get back on and continue my ride.