Why we have hair where we do. Insulation and, believe it or not, lubrication.
Nose hair: filters out large particles.
Ear hair: Keeps dirt and hopefully bugs out of the ears.
Under arm hair: Natural lubrication.
Groin hair: Same as under arm hair.
Ass hair: Same as groin hair.
Bodily hair: Remains of a natural temperature controlling fur coat. Even today, body hair responds to heat and cold by raising or compacting according to the temperature.
Head hair: Provides protection and insulation for the brain. Ever notice how slippery it can be? A poorly aimed blow would hit the hair and actually slide off doing less damage than if the skull was bald.
Beards: Facial insulation and protection. (Acts similar to head hair.) NOTE: The beard hair is different in texture and toughness from other bodily hair.
Later, head hair became a great beauty feature.
The diminishment of male head hair is possibly connected with aging when the older male, having survived beyond the violent teen and younger years, slows down and potentially no longer requires the body to expend energy and resources into protecting the skull. Genetics has a lot to do with it here and we could go into mutations caused by our mildly variable and radioactive sun, along with natural selection, but I will not.
Testosterone, of course, has a lot to play in the matter. The male needed more hair as the hunter gatherer and fighter while the woman needed less because her body has that layer of fat under the skin to provide her and any unborn child with insulation.
“Think of it as Evolution in action.”