What is the purpose of having Big Brother and Goldstein not appear in the novel, but showing such an influence on the people?

Big Brother and Goldstein serve as symbols in 1984: Big Brother represents absolute power, and Goldstein serves as a scapegoat. Both are fundamental to dictatorships. In having Big Brother appear only in the posters hung throughout Oceania, Orwell creates an awesome, awful presence in society--that of the omnipotent and allegedly benevolent dictator. Big Brother is the government that intrudes upon and controls every aspect of the individual's existence. So that the people have an outlet for their negative emotions and are therefore more easily controlled, the government gives them Goldstein as a target for their hatred. These two non-people exemplify concepts essential to dictatorships: that the people must feel safe, even though the government denies them their fundamental freedoms, and that the people must be inclined to blame someone other than the government for their problems.