Having shown us that no one can be saved through the Law (and even those who seek to be saved through a covenant of works will be lost) Paul now asks the question that this gives rise to: ‘Is the Law sin?’ His answer is that the Law is certainly not sin, but that when it comes in contact with our sin, our sin is enlivened. Paul perceived that he was alive in his sinful state, until the law came and showed him that he was dead before God.
We need the Law because it shows us our need for Christ. In evangelism we deal with many who believe they are alive. The Law is the God-given cure to this deception. Showing the Law to an unconverted person will often expose their sin and their death to them, showing them their need of a Savior. And even we who are saved need to be reminded through the Law of our great need for Christ.