As an example of what Paul is saying, consider the sixth commandment: Thou shalt not murder. Any community of people who are unaware of this law can be divided into two distinct groups: those who have no inclination to murder, and those who have varying degrees of inclination to murder, i.e. a price can be negotiated that will induce them to murder.
[![enter image description here][1]][1]
I have coloured the circles because I am aware of the Law, and those who have no inclination to murder are naturally righteous, and those who can be induced to murder are not.

Now, what purpose would be served by giving those in the green circle a law that says, "Thou shalt not murder!"? It would have zero impact on their inclination to murder. So, the law is given to those who are inclined to murder, to let them know that the Lawgiver values life, and it would please him if they did also.

Advertising the Lawgiver's pleasure to the citizens of the community (making a law) will create varying degrees of conflict within its citizens proportional to their respect for the Lawgiver and the degree to which they are inclined to murder. This conflict affords each citizen the opportunity to choose to pursue the Lawgiver's pleasure (righteousness) or their own pleasure (sin).

###Conclusion###
The illustration above is a simple scenario involving one law, which would get exponentially more complex as the number of laws increase. However, it serves to illustrate what Paul is saying.

The righteous are not released from the Law, they are bound to it by natural inclination. Telling them not to do something they have no inclination to do, or to do something they are naturally inclined to do anyway, is pointless.

The Law is for the unrighteous.

  [1]: https://i.sstatic.net/w0x05.png