In Orwell's work, the ruling class has to control the interpretation of the past. If the past is written to show anything but the consolidation of power on the part of the ruling class, the likelihood of a rebellion will be present. As part of the totalizing role of government that Orwell creates, nothing is left to chance. This would involve the control of the past. The narrative that has to be peddled is one where the ruling class knows best, that "war is the health of the state," and that there are enemies who must be exterminated. As long as the ruling class can control the past, it helps to bring credence as to why they are present and why they need to continue to be in the role of control. Few would have thought history to be so sinister, but Orwell's vision helps to create a perception of a discipline so often deemed as "boring" to have profound implications.
It is important for a number of reasons.
First of all, it is important because the Party needs to look like it never makes mistakes. So when someone who used to be in a position of power makes a mistake, they need to be deleted from history. If they were still there in the newspapers or in pictures, it would make the Party look bad.
Second, the Party needs to be sure people don't have memories of how things used to be. If they do, they might think that things were better before. Or they might at least remember that other things were possible. Both of those would be dangerous because they might make people more inclined to rebel.