What does Judges 3:4 mean?

This verse restates again why the Lord allowed all the nations listed in the previous verse to continue to exist. His original command to Israel was to entirely remove those cultures from the region (Deuteronomy 20:16–17). This was, in part, to prevent Israel from imitating those depraved, evil practices (Deuteronomy 20:18). Another was to punish the Canaanites for those very sins (Deuteronomy 7:1–5; 9:4–5). Since Israel refused to complete their mission (Judges 2:20), God chose to use the remaining Canaanites to serve His own plans (Judges 2:21–23). Even in their battles and wars against Yahweh's people Israel, they would prove and accomplish God's purposes.

God intended to use these nations and their oppression of His people as a test. When faced with oppression by foreign enemies, would God's people turn to Him in faithfulness for help or would they continue to stubbornly turn to false gods in hopes of being rescued by them? Naturally, God already knew the outcome of those events. This "testing" is in the sense of a demonstration: repeatedly showing the truth about how Israel would respond to their own sin and God's correction.