But wearing a helmet regardless if you hit the ground with some force head first it will save you some pain and trauma.....now with a car in which this happens rarely it can save you from getting your skull cracked open like a egg if you get hit and your head slams on the car or ground and your head slams on the pavement or smack the side of your head on the sidewalk or road.

In general you should at least wear a helmet when bicycling. At least have some protection while riding. It should be common sense.