Why did God bring Satan into the world?
God brought Satan on the scene so humanity could exercise the will to either respond to His love or reject it. The human race was made as objects of God's love, and Satan's presence provides the choice necessary for that voluntary response; otherwise there would be no exercise of will.
Les teaches from Ephesians 3:10 that God's manifold wisdom is now made known by the church to the principalities and powers in heavenly places. He explains that people ask why God made the human race if He knew it would rebel and end up in a mess. The answer centers on humanity 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 He had to bring Satan on the scene: so we could exercise that choice. Otherwise, there would be no will exercise.