The reason why doublethink is so central to Ingsoc is because doublethink is the way that the Party controls the thoughts of its population and makes them believe what the Party is telling them even though at some deep level they know that what they are hearing is not actually true. The reader is presented with numerous examples of this in the story, but perhaps the most obvious comes through Winston's job in the so-called Minstry of Truth, where he and other workers there deliberately and consciously change and edit public records to suit the Party's version of events at any given moment. Thus when an important alliance in the war shifts, Winston and all of his colleagues have to spend hours trying to edit history to erase the fact that the former alliance ever occurred. They know that this is false, and yet they come to accept it as a truth. Note how the novel itself defines doublethink:
The power of holding two contradictory beliefs in one's mind simultaneously, and accepting both of them... To tell deliberate lies while genuinely believing in them, to forget any fact that has become inconvenient, and then, when it becomes necessary again, to draw it back from oblivion for just as long as it is needed, to deny the existence of objective reality and all the while to take account of the reality which one denies – all this is indispensably necessary.
Doublethink is therefore so important in this dystopian society because without it the Party would not be able to present itself as infallible and all-knowing. Its errors and mistakes would show through, and as a result the people would lose their fear and respect of the Party. Doublethink is the very basis of the power of the Party and what makes it impenetrable, as doublethink is the reason for the control the Party has over its people.