Why does "1984" contain long passages from "the book"?
I assume you mean why does the movie version include long passages read by the narrator from the book itself. The reason for this is why I believe most movies can never match the book in intensity and meaning. When reading the book, we get into the heads of the characters. We know their thoughts, innermost feelings, fears, hopes, dreams, failures. It is very difficult to portray this accurately when portraying the characters in the movie without doing just what the director of the movie 1984 did--he gave us long passages of thoughts, feelings, motives, fears, etc. from the characters while also feeding us images and graphics which would help us grasp the overall message. It is nearly impossible to accurately depict characters in a deeper book (like 1984, Lord of the Rings, or even Harry Potter) in a movie that only lasts 2 hours. So much of the character's beliefs, opinions, feelings, thoughts, etc. is left out of the movie because of this difficulty. I was pleased to see that 1984's director decided to include passages from the book in an attempt to do the book justice.
Like the "newspeak" section at the end of the book, the lengthy extracts from Goldstein's book (if he is the author or even exists) on Oligarchical Collectivism tend to be skipped by readers. Nevertheless they are important to the novel as a whole.
Firstly, the extracts reinforce the reader's (and of course Winston's) belief in a genuine opposition to the Party; the belief is only really undermined much later, when O'Brien claims to have written much of it himself. Even then, we are left with the faint possibility that the Brotherhood and its aims really are meant by Orwell to be summed up in "The Book." (Though I don't believe that Orwell presents the Book as a serious political manifesto; even Winston notices that it isn't really telling him anything new. Some critics just see the extracts as a parody of Trotskyism.)
The extracts also give us a perspective that Winston, who has lived with the Party for most of his life, lacks; it fills in some gaps regarding how it came into being, and gives an idea of how the 1984 world functions.
Finally, it sets up a question; Winston's Readings is interrupted by the arrival of the Thought Police, just as he was about to read about the Party's motives. The question of "why" has long tormented him; it is answered near the end, brutally, by O'Brien: "The object of torture is torture. The object of power is power." The contrast between "Goldstein's" academic style and O'Brien's bluntness is very effective.