Back in Numbers 35:14 God had told Moses to establish "cities of refuge." In that context He also explained in more detail the protocol for protective entrance into those cities. For anyone who was guilty of manslaughter rather than murder, those cities were havens of protection. In the case of premeditated murder, each city would only function as a temporary holding cell until it was proved that the individual was worthy of death (Joshua 20:9). And, the cities were spread out far enough to be accessible everywhere in the nation. Since all the tribes had been assigned their lands, the only tribe left was Levi. Chapters 20 & 21 are devoted to describing where the Levites would be living. They were to be the inhabitants of the cities of refuge as well as other cities out of all of the tribes. Now, you should notice in this chapter that God commanded THEM to appoint which cities would be cities of refuge (Joshua 20:2). God could have appointed them, but He asked them to do it. And, they did (Joshua 20:7-8). They had a role to play in this program of mercy.
Why only 6 cities though? Why not 7 or maybe even 12? There were only 6 because the 7th city of refuge was still yet to come. Christ Himself is that 7th city of refuge. Of course, the great thing about the safety of being in Him is that His city was built for the guilty, not for the innocent. Well, in a way the 7th city IS for the innocent, but only because He graciously imputes innocence to us and removes our guilt. So, the New Jerusalem will be the true city of final refuge and we will live there safely until the death of The High Priest... which will be never!