Yahweh said to Joshua, Don't be afraid because of them; for tomorrow at this time will I deliver them up all slain before Israel: you shall hamstring their horses, and burn their chariots with fire.
There certainly appears to be more than one reason why God commanded Joshua to destroy the horses and chariots.  The necessity of doing so from the military viewpoint is quite evident, but there was also the further reason that God did NOT wish Israel to own any horses and chariots.  Deuteronomy 17:16 plainly warned Israel and their rulers NOT to go into the horse business, despite the fact of horses being in that period a prime element of military strength.  When Solomon multiplied horses (having forty thousand of them), it was displeasing to God.