From my my experience this usually differs depending on the type of book it is, but I think the best choice is either A, to help the reader understand new concepts, or C., It allows the reader to focus on key ideas and information during the reading. I'd pick C though, since it's more in depth and makes more sense.
B. doesn't make that much sense unless the book is an argumentative or a persuasive book.
D. only applies to personal narratives. Nobody can understand a writer's personal experiences and attitudes otherwise.