Why does "1984" contain long passages from "the book"?

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.