Rastafarians grow their hair into dreadlocks because it is a part of the Nazarite Vow. All Rastafarians take this vow and claim it is commanded by the Bible (Leviticus 21:5 "They shall not make baldness upon their head, neither shall they shave off the corner of their beard nor make any cuttings in their flesh").

See article "The History of Dreadlocks"