A couple of things I've read is that pubic hair is a lubricant of sorts that comes into play during sex.
Underarm hair - and pubic hair as well - was so infants and the very young would have something to hang onto when the mother needed all limbs
available for traveling or other things.
Facial hair for men may have come about because man was the hunter and probably exposed to cold and bad weather more than the female.
I agree with the comments about head hair being mainly useful to assist in regulating temperatures of the head.
As well as it being a sexual attractant.
Interesting part nowadays is that most women, when they get a little older cut their hair short.
I think that's more of a convenience thing than it is an age signifying device.
I've got a ponytail and taking care of it has been an education of sorts.
Mainly in appreciating what women go through with their hair.
Take a look at the fine hair on the back of a man or woman.
The hair lies in the same direction water would flow if the person was swimming.
There are theories that talk of mankind being aquatic and that's why the body hair lies as it does.
One other small one along the evolutionary front, women get cellulite and men don't.
That's because under the surface of the skin are fibers that cross one another very similar to a fishnet.
In men, the crossover points are close together so as to hold the skin together better under severe stresses - like being tossed down canyon by a
mammoth or similar.
Since women are the gatherers in early society their skin fiber crossover points are further apart because they don't need the rugged skin the men
Therefore, with the women having wider/further apart crossover points with their skin fiber, fat can protrude from between the crossover points more
than it does with men.
Which is . . . a shame since success in the gene pool depends on a womans physical beauty and all that's required from the men is that they be good
warriors and providers.
Sorta explains why every once in a while you see a stunningly beautiful woman with a guy who looks like he fell out of a coconut tree....