If you look at human physiology, helmets make a lot of sense.

The brain is very delicate. It's injured much more easily than other body parts, the things on your head are much more susceptible to permanent damage. Neural tissue, once damaged, does not grow back. Land on your arm heavily, you have multiple fractures, which can be re-set and healed. Land on your head heavily, and you have a subdermal hematoma that will likely give you brain damage.

When we ride bikes, we move faster than our bodies have evolved to protect... our brain cases evolved for the optimum balance of movement, weight and protection at running speeds over mostly yielding ground. With bikes, you move much faster over harder stuff. Yeah, there are always exceptions (dash your head on a rock) but the odds are much higher.

I know that's somewhat amateur physiology, but it seems like common sense to me. If you're going faster than a run, on hard material, it's wise to wear a helmet. If you're tooling along at walking speed on a beach cruiser, it's not really necessary.