Why is the theme truth important?

A common theme of dystopian fiction such as 1984 is complete and utter control of the populace; the powers that be will use everything to define the lives of their subjects, down to the language that they use (“Newspeak”) and the ideas that they are allowed to communicate. This is what makes truth such a central theme of George Orwell’s 1984; “Big Brother” is reliant upon his ability to define truth for the masses. The government states that “power is not a means; it is an end,” and regularly exerts its power over citizens in seemingly punitive ways.
We see truth as an essential theme of 1984 even to the point that there is a government agency known as the Ministry of Truth. The government defines knowledge, history, and language for their own benefit: “ ‘Who controls the past’, ran the party slogan, ‘controls the future: who controls the present controls the past.' ” The very existence of people can be decided by this Ministry of Truth: Winston is charged with creating an alternate life story for an individual that the government has deemed to be an “unperson.”
The common working man who is subjected to this control must never be allowed too much independent thought or advanced education, as Orwell notes in Chapter 1: “Left to themselves, they will continue from generation to generation and century to century, working, breeding, and dying, not only without any impulse to rebel, but without the power of grasping that the world could be other than it is.”
The extent of this mind control is evidenced in the language used within this society. The idea of “Crimestop” is meant to describe “the faculty of stopping short, as though by instinct, at the threshold of any dangerous thought.” The citizens are told what to think, what truth is and is not, and even made to accept contradictory ideas as logical (“doublethink”). In so doing, the government is able to communicate what they want to define as truth without being caught in their manipulative ways.
http://www.george-orwell.org/1984

https://www.nottingham.ac.uk/english/documents/innervate/15-16/02-olivia-rook-q33407-pp-14-28.pdf

https://www.timeshighereducation.com/news/wells-inspired-orwells-1984/159187.article