Many bike riders still see wearing a helmet as a recommendation, and they don't really need it because they are very skilled riders and other then that they know how to fall, and their skull is thick and strong, well that might be true but it wont save them all the time.
Unexpected malfunction You might have a very good control over your bike but when a wheel flies off, a tire explodes, frame fails, brakes do not respond you are in the mercy of gravity and inertia.
Unexpected obstacles when you ride you can't predict everything, so if you ride over 20 miles per hour you will encounter an unexpected obstacle sooner or later.
The damage could be intimidate or for the long term some brain injuries can be dramatic and cause lack of sensation in the ligaments or even permanent paralyses. Other injuries could not be noticeable immediately after the accident however the accumulated damage could have a devastating effect even year along the line, and may be interpreted in memory loss, dementia ,Alzheimer or Parkinson's disease.
This is what happens to your brain when your head gets hit