Here’s a list of some of the traditional rationales for the right to free speech (Eric Barendt for example has identified some of these in his book “Freedom of Speech“):
1. Freedom of speech serves the search for truth
There’s a long tradition in philosophy claiming that freedom of speech and the equal right of everyone to express himself or herself in public on any possible topic improves the quality of opinions and knowledge. Rawls, Mill and Kant for example have fleshed out this claim. In the words of Alexander Meiklejohn:
Just so far as, at any point, the citizens who are to decide an issue are denied acquaintance with information or opinion or doubt or disbelief or criticism which is relevant to that issue, just so far the result must be ill-considered, ill-balanced planning for the general good. It is that mutilation of the thinking process of the community against which the First Amendment to the Constitution is directed. (source)
Or in the words of Justice Oliver Wendell Holmes, in Abrams v. United States (dissenting):
The best test of truth is the power of the thought to get itself accepted in the competition of the market.
2. Freedom of speech serves individual self-fulfillment
People who can express themselves freely are better placed to develop their personality and identity. When you can say what you think and believe, you can better give shape to your thoughts and beliefs. Also, thoughts and beliefs depend heavily on the possibility to receive information, which is something that in turn depends on free expression. On top of that, persuasion is an important element of wellbeing: people who can persuade others feel better about themselves. And when they can persuade others, they can form communities and associations, and belonging is another important aspect of wellbeing and self-fulfillment. Finally, when the right to free expression is respected, people can better enjoy culture, education and other things that improve wellbeing.
3. Freedom of speech improves the functioning of democracy
Even for a minimal democracy (regular, free and fair elections for representatives) freedom of speech is very important. Candidates have to be able to advertise themselves and their policies and argue amongst themselves. Lobbyists should be allowed to make their case (publicly and transparently, of course). Etc. But democracy should be more than that. Ideally, democracy requires deliberation among the people on the best possible policies. It’s obvious that this deliberation requires free speech. More on democracy and free speech here.
4. Freedom of speech is a check on the corruption of power
People have to be able to receive information about the functioning of government. Free speech is a necessary prerequisite of government accountability. Freedom of information acts are just as much an element of free speech as a free press, and both are required to counteract corruption and abuse of power. At the margin, elements of free speech such as freedom of information, a free press and the right to protest can make the difference between freedom and tyranny, but they also limit the risk of lesser evils such as administrative corruption, betrayal of election promises, covert government activities etc.
5. Freedom of speech is a right that is required for the protection of other rights
Historically, it has been the case that other rights have depended on freedom of speech for their full protection. The civil rights movement and the struggle against racial discrimination in the U.S., for example, would have been impossible without freedom of speech (which doesn’t mean that the right to free speech of the proponents of equal rights was never restricted). Equally, the feminist struggle for equal voting rights for women was made much easier by freedom of speech. And finally, the right to religious freedom cannot be separated from freedom of speech. And there are many other examples.
6. Freedom of speech serves prosperity
Without freedom of speech there is less innovation and less trade. Scientists who develop new products or services need freedom of speech, and business people have a lot of difficulties trading or advertising without it. Hence, it can be said that economic growth is fostered by free speech. But free speech doesn’t only promote prosperity in general (on average); it also benefits the poor. The squeaky hinge gets the oil. If the poor aren’t able to make their case, they won’t get help.
More on freedom of expression.