Partly, to blot out the name and memory of so lewd and vile a people; partly, lest any of the Israelites should be tempted to settle there, and so be discouraged in their progress to Canaan; and partly, lest they should be possessed by other people who might prove as bad neighbours to them as these would have been.
Related Commentaries of Numbers 31:10
Numbers 31:10
10 And they burnt all their cities wherein they dwelt, and all their goodly castles, with fire.