10 which went to the descendants of Aaron, one of the clans of the Kohathites who belonged to the people of Levi; since the lot fell to them first.

Matthew Henry's Commentary on Joshua 21:10

Commentary on Joshua 21:9-42

(Read Joshua 21:9-42)

By mixing the Levites with the other tribes, they were made to see that the eyes of all Israel were upon them, and therefore it was their concern to walk so that their ministry might not be blamed. Every tribe had its share of Levites' cities. Thus did God graciously provide for keeping up religion among them, and that they might have the word in all parts of the land. Yet, blessed be God, we have the gospel more diffused amongst us.