In Orwell's 1984, why does government need to hide the truth from their people?
The government of Big Brother is predicated upon being able to control the will of its people. Part of this rests in ensuring that "the truth" fits its agenda. It is for this reason that the state of perpetual war is always reported and that citizens of Oceania are always being told that enemies exist everywhere. If the citizens, such as the Proles, got to know the truth, they could mobilize and challenge Big Brother's power. In his fateful last moments with Julia as they stare out the window at the Prole woman singing, Winston realizes that the Proles cannot be stopped by Big Brother if they mobilize.
The government of Big Brother understands and recognizes this. It is for this reason that they do wish for the people of Oceania to only know the truth that they provide them. This only "truth" is the that citizens need to be submissive to the government and that Big Brother's power is not questioned. Any other notion has to be hidden and controlled. "Truth" becomes a malleable quality in the hands of governments like Big Brother. It is used as something that can dictate, and its dissemination is always subject to "Ministries of Truth." The idea of "Truth" becomes one in which "a focusing point for love, fear and reverence" is evident. It is for this reason that governments that wish to control the message and the people conceal and hide truth, ensuring that what does come out is meant to consolidate the control that the state has over its people.
The government hides the truth from the people, to keep them from uprising. In 1984, the government hiding the "truth" was a way to keep people going through the motions of life without ever questioning the motives/actions of the government. It is also a way for the government to act in a corrupt/immoral way without being detected by its citizens.
On the flip side, governments hide "truths" from their citizens to protect them. Often their a national security issues that citizens can not know about, and the government hides these things as a safety precaution.
There are many ways that are used by totalitarian regimes to control and manipulate the people. The easiest one, however is to keep them in darkness and prevent them from thinking. To use Frantz Fanon's words, "colonizing their mind' is the best way to dominate and exploit them.