Here Paul addresses a mistaken notion people seem to have fallen into in his day. The notion seems to have been that if grace was a good thing to have, then sinning more would allow them to experience more of God’s saving grace.

Paul explains that if we die to sin—and it requires grace of God to do that—then more sinning is repugnant to that condition and inconsistent with a grace-changed life.

The assumption behind the error is that grace is only accessed to be saved from sin and that grace goes away after changing a person from a life of sin. 

Instead, God’s grace continues to abound in the changed life. And it must, otherwise there will be a fall right back into sin. The flesh continues to rebel against the spirit, and grace is needed for the spirit to win that battle every day.  Grace abounds when we humble ourselves and seek God’s help with our failings and weaknesses, when we pray to resist temptation, when we ask for strength in our fatigue, when we need and pray for more spiritual gifts to deal with new challenges and so on.