The chief office of the Law reveals original sin with all its fruit...

But the chief office of the Law or force of the Law is to reveal original sin with all its fruit.  It shows us how very low our nature has fallen, how we have become utterly corrupted.  The Law must tell us that we have no God, that we do not care for God, and that we worship other gods--something we would not have believed before and without the Law.  In this way, we become terrified, humbled, depressed.  We despair and anxiously want help, but see no escape.  We begin to be an enemy of God and to complain, and so on.  This is what Paul says, "The law came in to increase the trespass" (Romans 5:20).
~BOC, SA, III, II, 4