Are you trying to make responsive design scale for a complex site? Building an app, but your organization doesn't have an API yet? If so, you've probably got legacy content—content that already exists, and that doesn't fit neatly into your new project.
What do you do? You could ignore it and end up with one of those responsive homepages that devolve into big content blobs after just one tap, or a one-off mobile site that no one can remember to maintain. You could put it off until it becomes the bane of your existence: the thing that "breaks" your design, because it's way messier than you’ve planned for.
Or, you could deal with it. If you take the time to make existing content work for you—by understanding what you've got, identifying patterns and relationships in its structure, and cutting the cruft along the way—you'll end up with a system that will not just support your content, but _enhance_ its meaning, message, and power.