Paul raises the question his readers are bound to ask. If God's blessing comes by promise and not by the law, why did God give the law at all? Paul does not want them to conclude that the law was a mistake or useless.

Barnes says the point of the answer is that the law was not useless, but was given by God for great and important purposes. Gill also treats the question as an objection that needs an answer, insisting the law, though it cannot justify, is not worthless. Paul asks it so he can show the law's true and limited role.

Paul answers at once in the same verse: "It was added because of transgressions, till the seed should come to whom the promise was made." He adds that it "was ordained by angels in the hand of a mediator." In the verses that follow he explains that "the law was our schoolmaster to bring us unto Christ, that we might be justified by faith" (Galatians 3:24), and that "after that faith is come, we are no longer under a schoolmaster" (Galatians 3:25).

"Added" is the Greek prosetethē, it was added, coming after the promise to Abraham by "four hundred and thirty years" (Galatians 3:17). Gill says the "mediator" is Moses, who "stood between God and the people of Israel." Barnes reports that many take it of Moses and others of Christ, and he thinks it seems rather to refer to Christ, "the great Mediator between God and man." Moses himself said, "I stood between the LORD and you at that time" (Deuteronomy 5:5). Barnes notes that the Old Testament makes no mention of angels at the giving of the law (though he believes their presence is implied in Deuteronomy 33:2 and Psalm 68:17), but the New Testament does: Israel "received the law by the disposition of angels" (Acts 7:53), and it was "the word spoken by angels" (Hebrews 2:2). Gill connects them with the "ten thousands of saints" with whom the LORD came from Sinai (Deuteronomy 33:2).

The law had a real purpose, and also a time limit. It was given "because of transgressions," to show sin for what it is. Barnes puts it simply: "It shows people their duty. It reminds them of their guilt." Gill describes the law as restraining transgressions, detecting and making them known, and punishing them. Paul says something similar elsewhere: "by the law is the knowledge of sin" (Romans 3:20).

But the law was never meant to be the final word. It served "till the seed should come," and Paul has already said the seed is Christ (Galatians 3:16). Matthew Henry sums up its goal: "The great design of the law was, that the promise by faith of Jesus Christ, might be given to those that believe" (Galatians 3:22). God gave the law to lead people to the promise, not to replace it.