Ok, this is an obvious one, but I have to include it for completeness. It’s important for two reasons. First, women just tend to care more about things being clean (and have higher standards for what is considered “clean”) than most men.
Second—and this might be more important—women are always looking for clues about what kind of person you are. To you, a mustard stain might simply be a mustard stain. To her, it might be a sign that you don’t pay attention to detail, you don’t respect yourself, you don’t have social skills, or worse. Set her mind at ease and make sure both you and your house are in good, clean order.