Orwell's novel is essential about power and control--specifically, the frightening prospect of a totalitarian government controlling every aspect of society. One of the most important means of control exerted by Big Brother is that over what is known about one's history.
Winton's work in The Ministry of Truth is to "rectify" history in order to make Big Brother and IngSoc appear in a favorable light. This is very clear in Part One, Chapter Four. One example is Winston's creation of Comrade Ogilvy, who though fictional will "exist" due to Winston's account of him. The Party needed a hero and Winston manufactured one. Another is Winston's realization that while the Party is pleased to announce that the chocolate ration has increased, he knows that it has actually been reduced.
Essentially, the power to control information about the past allows the government to control the present. If you are sure that something happened--perhaps even witnessed it yourself--but no one acknowledged it, did it really happen? History is only relevant because we choose to make it significant. Big Brother goes one step further and manufactures history. The Party wipes clean any reference to what is not favorable and creates lies to suit its purposes.
Goldstein attempts to explain this in "The Book" in Part Two, Chapter Nine, but fails to articulate the full importance and significance of the mutability of the past and the essence of "doublethink." He can explain the "how" but not the "why."