It provides Sikhs a unique identity. You will see only Sikhs wearing turban in western or majority of eastern countries. If a Sikh wishes to become one with the Guru, he / she must look like the Guru (wear a turban). Guru Gobind Singh says:

Khalsa mero roop hai khaas. Khalse me hau karo niwas.
Kwlsw myro rUp hY Kws, Kwlsy mY hON kro invws]
Translation:
Khalsa (Sikh) is a true picture of mine. I live in the Khalsa.

According to the historical accounts, Guru Gobind Singh tied an 18 inch high dumala (turban) just before the light merged in the Supreme Light.

Sikh men commonly wear a peaked turban that serves partly to cover their long hair, which is never cut out of respect for God's creation. Devout Sikhs also do not cut their beards, so many Sikh men comb out their facial hair and then twist and tuck it up into their turbans along with the hair from their heads. Sikhism originated in northern India and Pakistan in the 15th century and is one of the youngest of the world's monotheistic religions. There are an estimated 18 million Sikhs in the world, with some 2 million spread throughout North America, Western Europe and the former British colonies. (SOURCE: ILLUSTRATIONS BY PAUL SCHMID / THE SEATTLE TIMES)