These nations were left to test the Israelites, to find out whether they would keep the commandments of the LORD, which He had given their fathers through Moses.
KJV
And they were to prove Israel by them, to know whether they would hearken unto the commandments of the LORD, which he commanded their fathers by the hand of Moses.
ASV
And they were left, to prove Israel by them, to know whether they would hearken unto the commandments of Jehovah, which he commanded their fathers by Moses.