Charles Spurgeon Commentary Galatians 3:17-19

"Now this I say: A covenant confirmed beforehand by God, the law, which came four hundred and thirty years after, doth not disannul, so as to make the promise of none effect. For if the inheritance is of the law, it is no more of promise: but God hath granted it to Abraham by promise. What then is the law? It was added because of transgressions, till the seed should come to whom the promise hath been made; [and it was] ordained through angels by the hand of a mediator." — Galatians 3:17-19 (ASV)

And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect. For if the inheritance is of the law, it is no more of promise: but God gave it to Abraham by promise. Wherefore then serves the law?

Some might argue that, as the law cannot justify, it is useless, but, on the contrary, it serves a very definite purpose, as Paul goes on to show.