Part of their government's control of the population depends on their ability to convince them that there is an "enemy" out there who threatens their happiness and security, and that the government, and ONLY the government, has the power to protect them from this enemy. It is important to keep this enemy before the people and even more important that they share some kind of emotional response to this evil person. In 1984, that function is served by Goldstein. He threatens them; they get together to shout him down ... only to have him replaced by the image of Big Brother ... the one who protects them from him. It does things on the emotional level that might not work as well on the intellectual level. If you want to see how this works, just watch the Republican or Democratic National Conventions next year ....
[If you want to see how this works in the novel, buy/rent the movie version of 1984 ... the depiction of the Two Minutes Hate is excellent.]
Another thing Goldstein allows Orwell to do, rather clumisly I think, is to introduce the theoretical underpinings of the opposition when Winston gets his hands on a "copy" of "The Theory of Oligarchical Collectivism." This allows him to get information in to the story that would be difficult to include without a subservice author to present the case in his text. It's similar to what Huxley does in Brave New World when he has the "lecture" at the beginning of the book when he explains how they came to be. Sometimes it's difficult for a dystopian author to get the "facts" into the story in an economical way ... Goldstein allows Orwell to do this.