John Calvin Commentary Joshua 21:20

"And the families of the children of Kohath, the Levites, even the rest of the children of Kohath, they had the cities of their lot out of the tribe of Ephraim." — Joshua 21:20 (ASV)

And the families of the children of Kohath, etc. Why it was necessary that the Levites should be dispersed among the different tribes, the reader may see in my Commentaries on the Books of Moses. This dispersion had, indeed, been imposed on their progenitor as a punishment for the cruelty and perfidy of which he had been guilty toward the children of Shechem, but its disgrace had been converted into the highest honor by their appointment as a kind of guardians in every district to retain the people in the pure worship of God.

It is true, they were everywhere strangers; but their role still possessed the very high dignity of acting as stewards for God and preventing their countrymen from forsaking piety. This is the reason for stating so carefully how many cities they obtained from each tribe; they were everywhere to keep watch, and preserve the purity of sacred rites unimpaired.