Humans still retain some body hair because they are useful. We lost most of our hair because we evolved in the hot open African savanna and we needed sweat glands, not hair, to stay cool and avoid heat strokes. The hair on top of our head is the exception, as it shields our head from the sun and at the same time curly hair allows a pocket of air to exist between the hair and the scalp. Breezes can blow through that air pocket to cool the head from time to time. The eyebrows allow us to keep rain from running into our eyes. The armpit and pubic hair trap phermones to attract the opposite sex. Hair don't grow in these areas until humans reach puberty and the endocrine glands in the armpits and pubic areas start to release phermones. Before puberty, no phermones are released and therefore no armpit or pubic hair is necessary.
The human body remained largely hairless except for the head armpits and pubic areas until about 40,000 years ago, when some people migrated to cold areas of the world in Europe and north Asia. At that time earth was in an ice age. There were earlier migrants out of Africa, but they ended up in tropical areas like South India, Australia and SE Asia. They retained the curly hair, dark skin and largely hairless body. People who migrated to cold areas evolved more body hair to stay warm. The way the body grew more hair was to increase the level of DHT (Di-Hydro-Testosterone), a male hormone. DHT increase facial and body hair. That is why European males have such lots of facial and body hair. Males from northern China Japan, and Korea also have more facial hair and body hair, but not to the same extent as European males. Other adaptations to the cold include straight hair (to eliminate air pocket between hair and scalp), light skin tones (to better absorb UV light for vitamin D production to prevent rickets), taller and narrower noses, thinner lips, shorter arms and legs and a more rounded torso to reduce heat loss to the environment.
High levels of DHT also has its negative side effects, as prolonged exposure to DHT can lead to shrinkage of the follicles, which leads to hair loss. People from Europe and north Asia therefore tend to be prematurely bald. Middle Easterners and many people living in the Indian Continent are descendants of Europeans, and therefore they too tend to have more facial and body hair. Some north Asians have also migrated south and interbred with people in SE Asia, so these people also tend to have lighter skin tones and taller noses than others around them who are more native, such as the Negritoes. People from West Africa, a cooler region than East Africa, also tend to have more facial hair.