In the book "1984" by George Orwell, why does the Party rewrite the past?

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