Nature has decorated a man with a beard and a moustache to differentiate between a male and a female.
A good example from the animal kingdom is a lion with its majestic mane.
Physical function: Just like the skin, the hair helps to synthesize vitamin D from sunlight. It also helps to supply the pituitary gland (located in the head) with phosphorous. Phosphorous is an element which is used in meditation by the aforesaid gland. The hair on our body regulates body temperature and our eye lashes, nostril hairs and ear hairs help to keep out dust particles.
Psychological function: This is by far, one of the most important functions of hair. People cut their hair to look good for other people, and although everyone wants to look sexy and cool, a Sikh is encouraged to impress God and not bother so much about the opinions of everyone else.
Keeping hair therefore encourages us to become less vain and more God orientated.
Sikh specific function: Uncut hair is a mark of Sikh identity. The 10th Sikh Guru instructed all his Sikhs to come before him with long hair and weapons. Long hair also represents sacrifice, because there have been many Sikhs like Bhai Taru Singh, who preferred to have their scalp removed instead of their hair cut.
Spiritual function: Hair enhances the ability of a human being to experience God. This can be explained by understanding the workings of electromagnets. An electromagnet consists of an iron rod with a coil of wire wrapped around it. The strength of an electromagnet can be increased by increasing the number of coils. Now in a human being, there are nine visible inlets/outlets (2 nostrils, 2 ear holes, 1 mouth, 2 eyes, 2 below the waistline). And the 10th inlet is located in the head and is invisible. It is called the Dasam Dawar.
This is where we experience the reality of God and we can consider this to be the iron rod. Hairs are like coils of wire which amplify spiritual energy at the 10th inlet. A greater quantity of head hair will lead to more coils in the (Joora) knot and therefore a higher concentration of spiritual energy. Of course, it is possible to experience God without any head hair like Buddhists. However anything that helps us to experience God more easily should be welcomed. Hair is essentially a spiritual technology that makes it easier to connect with God.
Related Article: Hair: A Biological Necessity by Dr. Birendra Kaur