I found this the other day that put it succinctly:

When we distinguish between God's ways in grace and His ways in government, we are not guilty of making a distinction without a difference. They differ widely.

Take as an example the case of the Apostle Paul. God's grace met him when he was in the flood-tide of his wicked opposition to Christ and His saints, he was saved, blessed and transformed. But this did not stop the working of God's righteous government on the earth, and many of his subsequent sufferings in the cause of Christ exactly suited his own previous misdeeds. He who "dragging off men and women, committed them to prison" (Acts 8:3) had his full share of imprisonment at the tie. He instigated the stoning of Stephen, and presently got stoned himself.

We needed the New Testament to give us the full revelation of the grace of God, but the Old Testament is of great value as giving us a large unfolding of the working of God's government. Observing His ways in the past, we are enlightened and warned as to what may be expected in the present; and we discover how amazingly apposite are these ancient histories. The Bible is indeed an up-to-date book.