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. To know whether they would hearken - This would be the consequence of the Canaanites being left among them: if they should be faithful to God, their enemies would not be able to enslave them; should they be rebellious, the Lord would abandon them to their foes.
Related Commentaries of Judges 3:4
Judges 3:4
4 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.