The introductory story took place with a horse and not a bike but it makes the point very clearly. Helmets aren't optional for safety but a requirement for responsible riders of both bikes and horses.
We were doing mounting exercises. I didn't have to wear a helmet but we decided to use it anyway. The horse spooked and took off with me half in and half out of the saddle. It was a very short wild ride before I came off and hit the ground hard.
When I came to I found out how lucky I was. The helmet had a three inch crack where my head hit a rock. If I'd skipped the helmet I would be dead.
Pressure : There is a poster hanging in my garage. It was given to my husband while he was working as a motorcycle mechanic as a young man. It has a picture of two broken eggs. It says "It only takes seven pounds of pressure to crack your skull."
When someone hits their head without a helmet at any speed the skull is in danger. When rocks are involved, which would be a probability on a mountain bike, that pressure is enough to do some serious skull damage … unless the rider is wearing a helmet. You might not get off completely uninjured. I didnt. You do have a better chance of coming out alive.
Location : My accident took place in a city. If we'd needed them, paramedics could have arrived in less than ten minutes and I was about that far away from a hospital. I would have been seen inside the "golden hour," the first hour after an accident. The golden hour is when the most can be done to mitigate injuries.
A mountain bike is generally ridden away from cities. The trails in my area are in rugged, mountainous terrain. Hikers and bike riders have had to be air lifted out when accidents have happened. Some have died. Of those, a few would have survived if they'd been wearing that essential piece of equipment.
Please take my story seriously. If you are participating in any sport make sure you are using the correct protective gear. If that includes a helmet, get the right one for your sport and wear it. It could save your life.