Jamieson, Fausset and Brown on Numbers 35:6

Nu 35:6-8. Cities of Refuge.

6. there shall be six cities for refuge, which ye shall appoint for the manslayer--The establishment of those privileged sanctuaries among the cities of the Levites is probably traceable to the idea, that they would be the most suitable and impartial judges--that their presence and counsels might calm or restrain the stormy passions of the blood avenger--and that, from their being invested with the sacred character, they might be types of Christ, in whom sinners find a refuge from the destroyer (see De 4:43; Jos 20:8).