Hair is needed for temperature control (insulation), skin protection from irritation (UV rays) and from things like rubbing such as under the arms. Once grown out hair is relatively inert. The components of hair need to be flexible and tough but is not a reserve for future use. Hair components are a vast compilation of various molecules. Being that they are not reabsorbed into the body there can be a way to eliminate some toxins from the body such as heavy metals and unwanted molecules.