What was the purpose of the Law for the nation of Israel?
The Law served as Israel's schoolmaster to bring them to Christ. For 1,500 years it was the system by which God trained, taught, and sought to mature Israel as believing Jews. Its purpose was to prepare the nation for the opportunity to be vessels and instruments used to reach the Gentile world.
Les teaches that in this passage the Law refers specifically to the Moral Law, the Ten Commandments, not the whole system of Judaism with its Ceremonial, Civil, and Moral parts. The Law was Israel's schoolmaster to bring them to Christ. For 1,500 years Israel was under the Law, and God used it as a system to train them, teach them, and hopefully bring them to maturity as understanding, believing Jews. That did not happen, but it was what God was striving for. The purpose of the Law was to prepare the nation of Israel for a great opportunity: to be vessels and instruments God could use to bring in the masses of the Gentile world.