Why is Orwell so concerned with history in 1984?
Orwell wants to show the potential power of the state to govern using control of information. One way that Big Brother does this is through rewriting history to comport with the Party's official version. This, indeed, is Winston's job at Minitrue. He destroys evidence that might seem to contradict the Party's version of history, and in so doing, contributes to Party control over the minds of the people. For example, while at work, Winston finds a photograph proving that three Party members executed for passing secrets to the leaders of Eurasia could not have done so. They were, the photo shows clearly in New York on the day they confessed to having been in Eurasia. He throws the photograph down the Memory Hole, where it is destroyed, and a tiny dent in the armor of Party orthodoxy no longer exists. By erasing people's memories about the past, and eliminating the possibility that they might gain accurate information about the past, the Party limits their ability to think for themselves, and makes them more susceptible to propaganda. This is why Winston realizes that "Freedom is the ability to say that 2+2=4." If people can only see, and publicly state, the truth, especially about the past, then the Party loses control over their minds.