Censorship is carried out in every country of the world and in various fields. Armies regularly view letters sent home by soldiers and delete any confidential or objectionable information. Governments regularly carry out censorship on many reports they make available to the public so that only watered-down versions come into public circulation.
Censorship in itself is not negative. It depends on the situation and the aim of it. In many authoritarian governments, such as the Syrian, Libyan, many African nations, China etc., the governments monitor and control all media and delete all objectionable material; such as dissent, complaints directed at the governing organs, complaints about the treatment of citizens etc..
It comes in many forms too, from the simple act of removing sections of text to the more sophisticated method of supporting, funding and aiding only those think tanks, researchers, experts, institutes and media outlets that are within the orbit of the governments national and foreign policies and mode of thought. This method of censorship nurtures an environment where only the supporters of certain ideologies will become the most successful and will receive the best platforms to launch their views and opinions. This method is carried out in every nation of the planet earth, while the other primitive and obvious methods are often carried out by the authoritarian regimes. IMO the effectiveness of the more subtle form is more efficient because it is less visible and noticeable. The constant debates of media objectiveness, fairness and balance come to mind.
Reporters Without Borders - International published a ranking of nations in order of the level of press freedom for 2002. You can view it
here. The results are quite interesting actually. The most press free nations are Finland, Iceland, Norway and The Netherlands, all of them sharing first place. The least press freedom is found in Africa and the Middle East, although Russia is just above Iran which rests at rank 122.
As to why persons might be in favour of censorship it would depend on what they have to loose. National secrets, military secrets, intelligence secrets, medical secrets, but also justification of rule and actions. The more there is to loose, the more will you encounter censorship.