Maybe not if a car hits you head on, but if a car cuts you off or clips one of your wheels a helmet will be useful.
A bike helmet has literally saved my life in multiple such occasions.
They don't protect from the part where the car directly hits the rest of your body straight on, or the bike itself. Every single part that happens immediately after that - such as the fall, flying through the air, or what have you, the parts that always come afterwards - is where the helmet can provide life-saving protection.
You swerve to miss a car, dog, obstacle, person and take a spill…