John Calvin Commentary Numbers 21:35

"So they smote him, and his sons and all his people, until there was none left him remaining: and they possessed his land." — Numbers 21:35 (ASV)

And the Lord said, to Moses. God first of all exhorts His people to confidence. He then commands that the men as well as the cities and villages should be destroyed, so that nothing should be preserved except the booty. He indeed addresses Moses only, but His injunctions are directed to all, because Moses, who was already sufficiently energetic, had not so much need of being spurred on as the others. God, however, had regard to the future also, lest the recollection of the blessing should be lost through the ingratitude of the people. In promising them victory, therefore, He desired to have the praise of it bestowed upon Himself.

I have already shown why He commanded the cities to be overthrown, and all the houses utterly destroyed: namely, lest convenient habitations should tempt the people to torpor, when they were required to hasten onwards to the promised rest; for those who had been ready in the wilderness to retire, and to go back into Egypt, would have eagerly taken possession of this fertile land and rested as in a delightful nest. By its desolation, therefore, they were compelled to abandon it. Its possession, indeed, was afterwards granted to the tribes of Reuben, and Gad, and half of Manasseh; but on condition that they should leave their herds there, and accompany their brethren through the whole expedition, not deserting them until the Canaanitish nations were destroyed.