The answers to this question will vary and you might have to sort through the various thoughts offered. One reason why free- thinkers are needed in a democracy is because the notion of individuals possessing power require a level of critical thought which is free to diverge from what leaders might say. "Democracy is not a spectator sport" is a line that reminds me of this. Individuals who are able to think freely and openly will be able to reflect on what is best for the nation, which will allow them to question authority. This means, when the establishment of power might be wrong or might not have the interests of the people at heart, free thinkers will be able to speak out and ensure that their government is responsive to their needs and the needs of the nation. Democracy, a form of government which is contingent on people and all individuals, is a system where individuals must possess the courage to speak their minds and think freely.
If you don't have people who can think for themselves, there is no point in having a democracy.
Democracy depends on having people who can think for themselves rather than believing everything they are told. In a democracy, we need to have people who can tell when the government is saying stupid stuff (like when Pres. Bush claimed we'd won in Iraq already or when Pres. Obama's people said that attempted attack on the airplane on Christmas showed that the safety system worked). If people can see through that kind of claim, they can sort of hold the government in check.
On the other hand, if they can't think for themselves, they'll just do whatever the government says. If they do that, we might as well have a dictatorship because we'll have no control over the government.
Does that help?
Freedom of thought is necessary for democracy because if not one would only have a collective controlled thought. If one votes fro something or a candidate then in order for the vote to be democratic choices must be present. Freedom of expression encompasses freedom of speech and media, freedom of thought, culture, and intellectual inquiry. Without them there could be no democracy.
In order to advocate for change there must be an open exchange of ideas and thoughts. It provides for all people, majority or minority, to be part of the decision making process in a democracy.
Full importance of freedom of expression could perhaps be appreciated only with the rise of totalitarian regimes, such as Adolph Hitler's Germany and Joseph Stalin's Soviet Union, among others.
In such regimes, the state not only exerted full control over expression, , it also used the media to direct citizens' thoughts and opinions through propaganda, indoctrination, denunciation, and social conformity.
In Fahrenheit 451, the body of government is totalitarian. Therefore, the above statement taken from the democracy website makes an excellent statement relevant to the book.