I suspect that Goldstein is an idea, the creation of the party to present a threat to their society, a common enemy for the people to hate, and perhaps someone they could use to ferret out potential enemies of the system as they do in Winston's case.
Having a common enemy is important for controlling the population. It provides an object for the "two minutes hate" and makes the population more and more dependent on the government for protection and safety. The Soviet Union once served as this object for us. It is interesting that, in our present situation, we have an unidentified enemy, and the inability to identify an enemy seems to hurt our ability unify the people in a common cause; this is easy in "1984."