This provision ensures that the leaders of the nation's faith and worship are dispersed throughout the land

 Joshua 21:1-19

Now the family heads of the Levites approached Eleazar the priest, Joshua son of Nun, and the heads of the other tribal families of Israel at Shiloh in Canaan and said to them, “The Lord commanded through Moses that you give us towns to live in, with pasturelands for our livestock.” So, as the Lord had commanded, the Israelites gave the Levites the following towns and pasturelands out of their own inheritance:

So the Israelites allotted to the Levites these towns and their pasturelands, as the Lord had commanded through Moses.

The total number of towns for the priests, the descendants of Aaron, came to thirteen, together with their pasturelands.

Total Israelites toke lot by Joshua for Levite have 48 towns. 36 towns in the west of Jordan river(13,10,13 towns from the southern, middle, north area), 12 towns in the east of Jordan.

The Levites receive no tribal inheritance: God is their inheritance. But they are given 48 cities, with pasture, by the other tribes. This provision ensures that the leaders of the nation's faith and worship are dispersed throughout the land.

Why?

Levites haven't land but have 48 towns distributed around every tribe's land. The Lord let them have places to rest with their family and distributed them around the whole Israelites. More important is to remind their faith in the Lord, let Levite be the center of their living. 

And the cities of refuge belong to Levite.

How?

God's plan and arrangement always is the best and complete. Trust in the Lord, God is good!