Why did God bring Satan on the scene?
God brought Satan on the scene so the human race could exercise its will, choosing to respond to God's love or reject it. Otherwise, there would be no opportunity for will exercise, as angels could not respond in that way.
Les explains that God’s eternal purpose, as mentioned in Ephesians 3:10, involves the church making known His manifold wisdom to principalities and powers. Addressing why God created humans knowing they would rebel, Les teaches it comes down to being objects of God’s love. Love requires a response, but angels could not respond. So God made the human race with an indwelling attribute of will to either respond to His love or reject it. That is why He had to bring Satan on the scene—so we could exercise that choice; otherwise there would be no will exercise.