Simply the advantages of integration. Lightroom lets me ingest files, keyword them, edit or add image data, sort, organize, review, archive, catalog, raw edit, image edit, and print, all in one integrated environment.
Conceptally, it functionally replaces Photo Mechanic, Iview Media Pro, Camera Raw, Photoshop and a RIP. Not all of them, not all of the time, but most of them most of the time. And it isn't even finished yet.