why does Orwell include detailed passages from Goldstein's book in 1984?

This is pretty long and drawn out section. I tend to skip this section when teaching this novel in favour of a simpler format. Basically the chapter addresses the three slogans of the Party. They are essentially paradoxes that serve the Party's autocratic ideology. Goldstein systematically takes them apart exposing what an educated reader already knows. "War is Peace " explains the all for one mentality behind conflict. As long as people do not question a war, the state is theoretically peaceful. “Ignorance is Strength” addresses the idea that if everybody stays stupid, they can be controlled and thus create a strong state. "Freedom is Slavery" addresses the idea that free will divides people and makes them vulnerable to outside attacks. Goldstein takes time exposing the party's intent of manipulating these slogans for their own ends.