Reading creates a lot of mind activity. You get ideas, questions or just want to put your new found knowledge on paper. When doing this reading books you would usually go to your notebook, in a device maybe you'll turn to "notes", or some other application within your phone. Finding these bits and pieces of thoughts is never easy. When you find them again, you don't really know the context, what book/article? Why?
Having a knowledge bank within the reading application makes this a whole lot more simpler. You just choose "note" and write down your thoughts, the application knows what book, where in the book and saves it all. When you would like to review the notes of certain book, you just go back, and skim through them until you have the "refreshment" you need.