What shall we say, then? Is the law sin? Certainly not! Indeed I would not have known what sin was except through the law. For I would not have known what coveting really was if the law had not said, “Do not covet.”
Most people do not like the law because they do not want to be punished for wrongdoing. This applies to those who break the law as well as for those who don’t. Watch everyone slow down as they pass a police officer on the highway regardless whether they were exceeding the speed limit or not to see my point.
Paul writes that the law is actually a good thing however. If boundaries were not established, he would be unable to know the difference between right and wrong. Certainly his conscience would guide him but people are capable of ignoring their conscience or justifying their actions. The law however is set in stone (quite literally). Do not covet means that coveting is not acceptable and the line can’t be blurred.
Because of the law, the boundaries for how one should live their life are clearly established. And because of this, Paul knows that he had broken those laws and hence had sinned.
Because the law points out sin, it also points out the need for a Savior. Without sin, Jesus didn’t need to die on the cross. But because Paul recognized that he was a sinner, he needed God’s forgiveness which was obtained through Jesus’ death on the cross.