You have been in this situation. You work on a user story, the story has been estimated for 5 story points. And you know how it goes, the team or the manager translated that to 20 hours of work. While thinking about the code that is needed, you see that existing code, created in previous user stories, needs to be refactored to accomodate the new bits. That is, if you want the code base as a whole to be of high quality.