Long time ago, everybody had two genders. One between the legs and one on top their heads. And we still carry around the proof; namely our hair. The pubic hair’s purpose is to keep things from getting in, and keep us warm. However, some porn actors and actresses who live in California can survive even without pubic hair. But I wouldn’t advise people to try this, as this requires an extremely mild climate.
But why do we have hair on our heads? Many men shaves their heads and can survive just fine, so it’s not the warmth. The reason is simple, thousands of years ago, we had two genders, with two different purposes. One was for re-procreating, and one was for peeing. The human race however, became lazy and didn’t want to have sex standing on their hands, but also there were hygienic issues to (people got poop on their heads).
Evolution took care of this for us, and eventually the gender between our legs took care of both peeing and re-procreating, and the one on our heads started to disappear. But we still got hair on our heads. Some people even got a small gender like growth on their heads, covered with pubic hair, but we just call it “hair”.