Albert Barnes Commentary Numbers 35:6

"And the cities which ye shall give unto the Levites, they shall be the six cities of refuge, which ye shall give for the manslayer to flee unto: and besides them ye shall give forty and two cities." — Numbers 35:6 (ASV)

The Levitical cities were in a special manner the Lord’s; and therefore the places of refuge, where the manslayer might remain under the protection of a special institution devised by divine mercy, were appropriately selected from among them. No doubt also the Priests and Levites would be the fittest persons to administer the law in the doubtful cases which would be sure to occur (compare Numbers 35:24 note).