Adam Clarke on Genesis 47:21

And as for the people, he removed them to cities - It is very likely that Joseph was influenced by no political motive in removing the people to the cities, but merely by a motive of humanity and prudence. As the corn was laid up in the cities he found it more convenient to bring them to the place where they might be conveniently fed; each being within the reach of an easy distribution. Thus then the country which could afford no sustenance was abandoned for the time being, that the people might be fed in those places where the provision was deposited.