Giani Kulwant Singh provides a simple explanation on why Sikhs cover their heads. Many people with atheist views are diluting Sikhi and questioning practices that have been going on for centuries.
All Sikh Gurus since Guru Nanak Dev Ji wore turban. However, the covering of hair with turban was made official by Guru Gobind Singh Ji, the tenth Guru of Sikhs. The main reasons to wear turban is to take care of the hair, promote equality, and preserve the Sikh identity.
Sikhs do not cut their hair as a respect towards God and the turban protects the hair from dust. Some people might ask the question that if Sikhs cover their hair on head, why don’t they cover their beard? Dirt is not a big problem for beard. The beard can be cleaned very easily while washing the face. Covering of the head by turban also symbolizes respect towards God. People form many other religions including Hindus, Jews, and traditionally many Christians including Catholics cover their head while visiting their church (Source: Christian Head Covering). Since Sikhs believe God to be present everywhere, they cover their head not just in church but everywhere else as well.