2Cor3 :7-11 (NIV) : Now if the ministry that brought death, which was engraved in letters on stone, came with glory, so that the Israelites could not look steadily at the face of Moses because of its glory, fading though it was, will not the ministry of the Spirit be even more glorious? If the ministry that condemns men is glorious, how much more glorious is the ministry that brings righteousness! For in comparison with the surpassing glory, how much greater is the glory of that which lasts!
The new covenant is more glorious than the old. Only the new covenant can make us righteous through Jesus. The old covenant(law or letters on stone) brings condemnation and death as it can only show us how unrighteous we are because not one of us can ever follow perfectly the requirements of the law. Yet, the old covenant is helpless to rescue us from our unrighteousness. The old covenant is like a mirror showing how ugly, unrighteous and sinful we are and yet unable to lift a finger to redeem us as it is only a mirror. In the new covenant, Jesus came in person to die on the cross to rescue us from our sin.
The function of the old covenant(law) is to show how unrighteous we are so that we can come to an end of ourselves and turn to Jesus for help.