Adam Clarke on Joshua 21:1

The heads of the fathers of the Levites - The Levites were composed of three brand families, the Gershonites, Koathites, and Merarites, independently of the family of Aaron, who might be said to form a fourth. To none of these had God assigned any portion in the division of the land. But in this general division it must have been evidently intended that the different tribes were to furnish them with habitations; and this was according to a positive command of God, Num 35:2, etc. Finding now that each tribe had its inheritance appointed to it, the heads of the Levites came before Eleazar, Joshua, and the chiefs of the tribes who had been employed in dividing the land, and requested that cities and suburbs should be granted them according to the Divine command.