3 Purposes of Law

A knowledge of God's Law serves three purposes.

  First, the Law sets before us the righteous standards of God.  We see what it is to live righteously  in a way that pleases God. [...]  (Romans 7:12)  The Law shows us what God expects from us.
  The problem is not the Law. We are the problem! We are unholy, unrighteous, ungodly sinners.  So even when we try to live according to the Law, we sill always break it in one way or another.

  Thus, we come to the second purpose of the Law: By the Law is the knowledge of sin.  When we know what God expects and don't live up to it, we see our sin.  As you teach your children the commandments of God, they will see that they fall short of God's standard.  This understanding brings the knowledge of sin.  It is the sinners who need a Savior.  Jesus came not to call the righteous, but sinners to repentance.
  John Newton's knowledge of the commandments of God and the consequences of his sin brought him to the grace of God.

  The Law also serves a third purpose, if it is heeded.  Is serves as a tutor, a schoolmaster to bring us to Christ.  If John Newton had listened to his mother and honored all that she stood for and taught him, it would have saved him much grief and remorse.  He failed to allow the Law to be his schoolmaster.
  As you bring up your children under the Law of God, you are putting them under a schoomlmaster that will show them their need for the grace of God.  If they heed the Law, it will serve as a guard against a life of blatant sin and the bitter consequences that come from transgressing God's holy commandments.  [...]
  Teach your children the difference between right and wrong -- the Law -- but keep telling them of the grace of God that can be theirs through Jesus Christ and the New Covenant. 

~  "Lord, I need Grace to Make it Today" by Kay Arthur, pg. 48-49