In the book "1984", what was the motivation to constantly tweak and alter the past?
"To the winners goes the spoils of war." Also, the winners write the histories. That permits them to show cause and effect with regard to past actions showing these were indeed the correct actions to take. Only by restricting the flow of information can this be accomplished; if individuals are free to investigate sources, the truth will eventually be told. But if the past is constantly altered to suit the whim of the present, nothing can be verified, and the only "truth" that can exist is what the authorities say is true.
Controlling and rewriting history allows The Party to control the minds of its members. They can say that life is better now than it was before they came into power, without the accountability of verification. In some ways this is what all political parties desire to do: promote their own ideals and subvert that which would harm their image.
In a larger sense though 1984 demonstrates an idea that history can be rewritten, in that it exists only in written documents and the memories of those who lived through it. Party members don't really care what did happen, only in what the Party says happen. It demonstrates the idea that there is no objective truth, which is the very foundation of doublethink.