What good do you derive from the knowledge of this fact?

Very Much. It would go ill with us if devils and wicked men had power to do anything in spite of the will of God. Moreover we could never be at rest in our minds if we were exposed to them in danger, but when we know that they are curbed by the will of God, so that they can do nothing without His permission, then we may rest and breathe again, for God has promised to protect and defend us.