The benefit of the law – guilt.

John Calvin has written:

“There are many passages in Augustine, as to the utility of the law in leading us to implore Divine assistance. Thus he writes to Hilary, “The law orders, that we, after attempting to do what is ordered, and so feeling our weakness under the law, may learn to implore the help of grace.” In like manner, he writes to Asellius, “The utility of the law is, that it convinces man of his weakness, and compels him to apply for the medicine of grace, which is in Christ.” In like manner, he says to Innocentius Romanus, “The law orders; grace supplies the power of acting.” Again, to Valentinus, “God enjoins what we cannot do, in order that we may know what we have to ask of him.” Again, “The law was given, that it might make you guilty—being made guilty might fear; fearing, might ask indulgence, not presume on your own strength.” Again, “The law was given, in order to convert a great into a little man—to show that you have no power of your own for righteousness; and might thus, poor, needy, and destitute, flee to grace.” He afterwards thus addresses the Almighty, “So do, O Lord, so do, O merciful Lord; command what cannot be fulfilled; nay, command what cannot be fulfilled, unless by thy own grace: so that when men feel they have no strength in themselves to fulfil it, every mouth may be stopped, and no man seem great in his own eyes. Let all be little ones; let the whole world become guilty before God.”

(Calvin, John. Institutes of the Christian Religion, Grand Rapids, MI: Wm. B. Eerdmans Publishing, 1998, p. 306-307.)