What effect of the quote "whoever controls the past controls the future and whoever controls the present controls the past" did it have in 1984?

The mutability of the past is one of the key ideas brought
out in "1984".  This means that the past can be changed to reflect the ideas of whomever
is in control.  The controlling powers can then make people believe that things happened
that didn't really happen, thus giving the illusion that the controlling powers were
always right and always good.  Winston Smith's job in the Ministry of Truth in the story
was to do just that - rewrite the past.  He would receive little tidbits of old news
stories to rewrite so that the stories now reflected what the Party in Oceania wanted
people to believe.  The quote you refer to supports this idea of the mutability of the
past.  In a power can alter what people believe happened in the past, then they can
control people how people think and behave in the future because of these altered
perceptions.  In order to alter the written history, a power must be in charge in the
current day, thus "...whoever controls the present controls the past."  Oceania
constantly changed the past to make themselves look good.  They would change the facts
about who they were at war with.  One day. it might be East Asia, the next, Eurasia.  If
they were at war with Eurasia, then the Party would rewrite the past to make it say that
they had always been at war with them and that East Asia had always been their ally. 
The next day, the opposite might appear in historical texts and newspapers if the war
had changed fronts.  Oceania might change their predictions, i.e., they might predict
that wheat production would be at 60% and when it actually turned out to be 40%, history
would be rewritten to show that their prediction had actually been 40% and they were
right all along.