With a helmet absorbing energy tranfer to the head from impact, your brain, which is basicaly floating in fluid and surrounded by a skull of solid bone. Without it, your skull is unforgiving during an impact, and the brain hits it inside on impact...that is what a concussion is.
Think football, many great quarterbacks quit after one or two concussions, pretty sobering when they run on a grassy field at maybe 10 miles an hour with pads and helmet.