Here’s a great Reformation treatment on the purposes (or uses) of God’s law.
“What purposes does the Law then serve?”
First, the Law helps to control violent outbursts of sin and keeps order in the world (a curb).
Second, the Law accuses us and shows us our sin (a mirror).
Third, the Law teaches us Christians what we should and should not do to live a God-pleasing life (a guide). The power to live according to the Law comes from the Gospel.”
That’s worth committing to memory: the law is a curb, a mirror, and a guide for the Christian to follow by the power of the gospel. Even young children can understand that!
This Q/A can be found in Luther’s Small Catechism with Explanation (Saint Louis: Concordia Publishing House, 1991). As I’ve mentioned here before, this is a nice little hardcover book that goes through the basics of the Lutheran side of Reformation theology. Even though I disagree with some aspects of Lutheran theology, this book is a great one to own and read.