Humans are mammals — just like cats, dogs, bats, elephants, monkeys and foxes. Ever notice that mammals are pretty hairy animals?
That’s because mammals have to stay warm — and having a a hairy fur coat helps. (Even though, depending upon what kind of mammal you are, you have to work hard to keep your fur coat clean!) A fur coat provides great insulation.
Additionally, if a mammal is scared, hair helps in looking big and fierce. For example, if a cat is scared, a muscle attached to the base of each hair can tug on it to make it stand on end. The result? The cat looks bigger, and with hair standing on end, scarier!
It’s unfortunate that humans just don’t have enough hair to be terribly useful. For the most part, hair ends up simply being something that makes for a good or bad hair day. However, hair IS responsible for goosebumps which occur when cold air or fright cause your hairs to stand upright making little bumps as they do. Cool, huh?
Answer provided by Discovery.com