If you really don't know why a helmet is a lifesaver, I can only suggest you haven't really studied the ergonomic nightmare that is the cockpit from a "what would really spoil my day if I headbutted it during a 10G deceleration" point of view. My personal vision on the road to Damascus was from some years ago, when a Hughes Pilot encountered resonance on a Ground run, the aircraft rolled over and the Gyro caging knob punctured his temple and killed him. Must still rate as one of the most avoidable and pointless fatalities in aviation for want of a bone dome.