People depend on their understanding of their history (the past) to make judgments about their present. If the party can control the past, tell people what WAS in the past (even though it wasn't), they can control what people think about now and the future. A simple, if political, example will make this clear in our own time. We have been told by our present administration that the Bush administration caused just about every problem that we are presently experiencing. In order to correct this we MUST do the things they tell us. Is any of this true? Did Bush make the mistakes that led to the situation we are in? Is the economy under anyone's control? If it were, wouldn't they have "fixed" it so that they could hold on to their jobs?
Most of us don't know the answers to these questions; we might not even be able to find them. But if we believe what we are told without supporting evidence, then we can be led to believe almost anything. There are many thing that I/we will never know about for sure; if these gaps can be filled with anything the government wants, then we can be made to believe almost anything ... and act on that belief.
For the party, the past is just a tool to control the present. They rewrite it to serve any purpose they wish.
"The price of freedom is eternal vigilance." Thomas Jefferson
The Party rewrites the past because "if you control the past, you control the present." In his novel "1984,) George Orwell demonstrates people can be controlled through cultural conditioning. Because people will put their faith in a government that they believe tells them the truth, a Ministry of Truth is created. This is where the main character, Winston Smith, is employed. There his job is to alter or "rectify" all past news articles that have been proven to be "false." However, it is Winston's need to reconcile what he knows with the Party's version of Reality that causes his nemesis.