What is the true purpose of censorship? How has it been used for good?

[Jay]

The true purpose of censorship is to present one side of an argument and ignore anything that is contrary or contradicts that point of view. Generally it exists to back up a government.

Censorship can sometimes be used for positive reasons - in WWII it was imperative to print good news and bolster public opinion for the war. News on reverses and defeats can affect morale so perhaps some media massaging is valuable to ensure that there's not too much negativity.

Censorship can also be used to quash ill-founded rumours and in some cases sensitive acts of the government are censored because the consequences of their being broadcast might be harmful. For example, suppose a newspaper discovered that the US government knew where Osama bin Laden was hiding and was planning to capture/kill him then it would jeopradise the mission by publishing or broadcasting a story about it. However governments should never be above criticism or media investigation.