One thing I've become aware of is how hair can serve as a health indicator. I've noticed that if I increase the fat content in my diet, my hair feels healthier. I think hair is a health indicator that is underused by the medical profession. Couldn't a hair analysis be a part of a health checkup? I bet it would provide some very valuable health information.
I read somewhere that there is an evolutionary reason for women to have long hair. The idea was that long hair provides a "track record" of a person's health over the last couple years. If a young woman has nice long hair, it is a clue that she has good resistance to infection/disease, and therefore would make a good mate. You also often see an age difference in hair length with young women favoring longer hair, and older women having short hair.
You can also look at the hair product industry and see how much people are willing to spend for good hair. Especially for women's hair care products, I would guess the annual sales are in billions. Certainly, hair is more than just a covering for the top of the head. In Paleolithic times, hair was probably a key indicator of access to quality food and resistance to disease.