A boomerang acts like a wing, which is why it flies. But the curve of the boomerang wing, combined with the spin given by throwing it holding from one end, causes the air flowing over the boomerang to twist it, and to put it into a curving flight path. Thrown right, the boomerang hits its target. (It was originally a hunting weapon.) Thrown almost right, it makes a large circle. It comes back to the thrower so he can try again without running to pick it up. Beats a spear any day.