First imagine what the motivation would be for a government to limit these things? They may claim it's national security, but in the extreme, such as the limits imposed in Bradbury's fictional society, it is almost always as a method of government control of society. Free speech means tolerating dissent, and dissent can cause both violent and non-violent revolutions, or simple widespread acts of rebellion. Look at what's happened recently in the Middle East, and the role cell phones and Facebook have played in this.
Protecting the right to free speech is what allows the people to speak freely (and be heard) by others and by their government. (This has already been posted). There is also the more general epistemological reason. And that is, if there is no free speech, or if speech is limited to an owning class, aristocracy, the state, or any other elitist group, then the basic communication of ideas is also limited. The right to free speech provides the imperative opportunity of each citizen to challenge the government, thereby keeping them in check - just as their function is to keep certain establishments in check, and to sustain - and improve - equality (this is a learning process).
The epistemological imperative is that without free speech, the sharing and dissemination of ideas is limited to a certain group (usually a viewpoint of narrow ideology). A good example of free speech in practice are blogs where people of all disciplines share ideas (literature, science, etc.), theories and practical applications. Free speech is usually cited as imperative for the voice of political dissent but it's also imperative for the evolution of other disciplines.
This might be another question where multiple queries will represent the findings offered. Freedom of speech is critical in any political setting and vital in a democracy. It is not surprising that the liberty of free speech and personal expression is the First Amendment to the United States Constitution, and represents one of the most elemental notions of personalized ideas of freedom. Its protection is critical because it allows the citizenry to voice their support or discontent without fear of reprisal. The ability for any citizen to be able to articulate their true feelings about government and legislation is essential for a government to be legitimized and to possess a sense of validation from its people.
The right of free speech has been called "the guardian of every other right" because it is so important. It is the right that protects us from having our government go bad.
In our system, if the government starts doing things we don't like, we get to speak out as loudly and angrily as we like. If enough of us are angry enough, the government knows that it has to back off. By speaking out, then, we can keep the government from doing things we don't want it to do.
That is why free speech is so important. If we could not express our unhappiness with things the government does, it would be able to do whatever it wants and we would have no control over it.
Without freedom of expression/speech there can be no democracy. It is the foundation of democracy. Democracy was founded under the ability to be able to share ideas openly and to make decisions based on the freedom of the exchange of ideas. If a person is silenced then he or she can not express an opinion, share ideas, or engage in appropriate voting. In reference to the book "Fahrenheit 451" one should note that taking a look at totalitarianism helps one to better understand why speech should not be limited.
As a society the people must protect their rights or they will lose them.