What is the purpose of two minute hate in 1984?

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.