Christians have to attend to the mastery of their sinful impulses so that they can serve their neighbors effectively. Luther approached the issue of sanctification with some reserve for fear that his generation, so recently weaned from the idea of good works as a means to grace rather than the fruit thereof, would fall back into the old patterns of thought.
One of the many practical effects of the Reformation was a change in how people viewed what are now called “spiritual disciplines.” In the context of the Reformation, most people believed that their good works contributed to their justification. They also believed that doing super-spiritual things like becoming a monk/nun would be rewarded by God. However, as the Reformers taught and preached the truths of Scripture, slowly but surely, people began to understand that their good works did not help in the matter of justification. Furthermore, they began to understand that super-spiritual things like monasticism were not found in Scripture and therefore could be abandoned. However, the Reformers noted that good works and biblical spirituality were still certainly part of the Christian life. Here’s a good summary of how Luther emphasized good works and the “spiritual discipline” of serving one’s neighbor.