Waterfall bookcase is a great way to break up the monotone look of library walls with a graduated shelves that are reminiscent of cascading waterfalls. On a functional level, waterfall bookcases are a great way to arrange books by size, while making sure that smaller books do not end up lost on uniformly deep shelves. Besides, this helps fight the urge to have more than one row of books on each shelf - a plague of many home libraries!
In modern design, ladder bookcases are most reminiscent of traditional waterfall bookcases so prominent in Regency and Victorian libraries. I would not be surprised if some people used the term waterfall bookcase loosely, thus including ladder bookcases, but the latter (pun intended) also have the effect of preserving the openness of a modern living room, they do not dominate the space and allow the color and detail of their surroundings to remain visually prominent.