Goldstein identifies Big Brother as the ultimate face of the Party. It is as if he suggests it is possible that Big Brother doesn't exist. It is understood that Big Brother never dies, so even if there is just one man in the position that rules, he is replaced upon death to keep the life of the Party alive.
Big Brother is the guise in which the Party chooses to exhibit itself to the world. His function is to act as a focusing point for love, fear, and reverence, emotions which are more easily felt towards and individual than an organization.
Big Brother is obviously the personification of the organization of the Party in order to address the human nature of people. Although the effort of the Party is to dehumanize mankind, they recognize what propaganda tools they must use to communicate with their subjects. Big Brother sits atop the pyramid of society as the ultimate authority and leader of the Party.