Galatians 3:19-20

What, then, was the purpose of the law? It was added because of transgressions until the Seed to whom the promise referred had come. The law was put into effect through angels by a mediator. A mediator, however, does not represent just one party; but God is one.

(v19-20, NIV)

The purpose of the law is to provide a standard. With this standard, we can properly measure ourselves against it to see whether we have sinned or not. Of course, we must remember that is it by grace through faith that we are saved and not by the law. An interesting point to note is the fact that there was sin before the law. However, without the law, sin is not counted (Romans 5:13). So, the law was necessary to show our sinfulness and to reveal God's righteousness.

A commentary I read online used the idea of a city with many traffic accidents, but no traffic laws. There would be many dangerous drivers, but it would be difficult to determine what constitutes as crossing the line from safe driving to harmful driving. The police and law enforcements would be unable to properly arrest people or write them tickets for "illegal" or unsafe driving.

The word "added" reveals a temporal nature to the law. This is contrasted with the permanent validity of the promise. So, the law becomes an added supplementary, secondary to the permanent covenant God made with Abraham.

This next part is described as quite difficult by one of the commentaries I read: "historically, this has been a most perplexing verse with many different interpretations." However, we can always try. There is this matter of the mediator. God has given the law through the angels to Moses. The mediator is Abraham and the intermediaries are the angels. The point Paul makes is another one showing the inferiority of the law. There is a contrast between intermediaries and God. There are many intermediaries, but there is only one God. The contrast is then applied to the promise and the law. The law was given from God to the angels to Abraham. However, the promise was simply given by God and God alone.