Why did God bring Satan onto the scene?
God brought Satan onto the scene so the human race could exercise its God-given will—choosing to respond to God's love or reject it. Humans were made as objects of God's love, and genuine love requires a voluntary response; without Satan, there would be no real exercise of that choice.
Les explains that God's eternal purpose is connected to His manifold wisdom being made known by the church to principalities and powers in heavenly places. When people ask why God made humans knowing they would rebel, the answer centers on humans being objects of God's love. To be an object of love, the giver expects love in return. Angels could not respond, so God made the human race with the indwelling attribute of will to either respond to His love or reject it. That is why God had to bring Satan on the scene—so humans could exercise that choice; otherwise there would be no will exercise.