There's much in this thread about the helmet absorbing the energy of the impact, but that's far from the whole benefit of wearing one.
The helmet, by conforming to the shape of your head, distributes the energy from the point of impact to a much larger area. So instead of having your head cracked open at the point it hits the tarmac, or pierced by a pointy rock, the force will be evenly distributed across your head.
The helmet also has a low-friction shell that will slide across a surface, which helps avoid head rotation, snapping your neck or getting whiplash, and will protect you from cuts and abrasions.
The helmet, by conforming to the shape of your head, distributes the energy from the point of impact to a much larger area. So instead of having your head cracked open at the point it hits the tarmac, or pierced by a pointy rock, the force will be evenly distributed across your head.
The helmet also has a low-friction shell that will slide across a surface, which helps avoid head rotation, snapping your neck or getting whiplash, and will protect you from cuts and abrasions.
I've been saved by my helmet many times.