The job of the Ministry of Truth, where Winston works, is to manipulate history. Each person who works there rewrites what happens according to the Party's instructions. By controlling what everybody in society believes to be true about where they have come from and the events which have helped to shape them, they can control what people believe and how they behave. If there is no access to real truth, then there is no way for people to learn from their mistakes and avoid repeating history. This is exactly what the Party wants.
Winston does as he is told, even as he is not sure how he feels about it. When 1984 begins, he has a vague ambivalence toward his job. In some ways, he realizes that they shouldn't be rewriting reality, but he goes along with it anyway, mostly unquestioning, as it is his duty.
When Winston does try to find out more about what society was like before the Party was in rule, he has a hard time finding answers. He asks one man, who is reluctant to share what he knows. Winston realizes that because the people who know the truth are unwilling to share it, there is no way for their population as a whole to have access to the information that they would need to live independently. Everybody continues the status quo so as to avoid danger. Without the stories and legends of those who have tried to fight the Party to guide them, there is very little risk of a rebellion.