Most of the time Luther is busy talking about how nasty the Law is, to make sure we don’t make it our hope. But when he gets around to talking about its good purpose, he can say some very nice things. The “final cause” of a thing, by the way, is its end, goal or telos:

The final cause of the obedience of the Law by the righteous is not righteousness in the sight of God, which is received by faith alone, but the peace of the world, gratitude toward God, and a good example by which others are invited to believe the Gospel. (Comm. on Galatians, 373)