Adam Clarke on Joshua 21:4

Out of the tribe of Judah - Simeon, and - Benjamin, thirteen cities - These tribes furnished more habitations to the Levites in proportion than any of the other tribes, because they possessed a more extensive inheritance; and Moses had commanded, Num 35:8, From them that have many, ye shall give many; and from them that have few, ye shall give few: every one shall give of his cities unto the Levites, according to his inheritance. It is worthy of remark, that the principal part of this tribe, whose business was to minister at the sanctuary, which sanctuary was afterwards to be established in Jerusalem, had their appointment nearest to that city; so that they were always within reach of the sacred work which God had appointed them.