Countries like China or Germany don't have free speech and are doing fine in practical terms (GDP, crime rates, political stability, global power). What is the advantage of having free speech in countries like the US? Why have the first amendment?
For practical value, the best argument is in J. S. Mill's On Liberty. Censorship assumes infallibility. The best way to make sure you are not mistaken is to allow yourself be challenged by opposing views.
He who knows only his own side of the case knows little of that. His reasons may be good, and no one may have been able to refute them. But if he is equally unable to refute the reasons on the opposite side, if he does not so much as know what they are, he has no ground for preferring either opinion... Nor is it enough that he should hear the opinions of adversaries from his own teachers, presented as they state them, and accompanied by what they offer as refutations. He must be able to hear them from persons who actually believe them...he must know them in their most plausible and persuasive form.”
― John Stuart Mill, On Liberty
China is a living example of why freedom of speech matters. In 1956, Mao started a hundred flowers campaign in which he encouraged freedom of of speech. He thought people would attack his political rivals and sing praises of himself, but he was wrong. People directly criticized Mao himself. Mao quickly noted down who spoke against him and started anti-rightist movement in which he sent whose who spoke against him to hard labour. The Chinese people learned a lesson and dared not to contradict Mao again. In 1958, Mao launched his Great Leap Forward; it was a disastrous economic campaign but no one dared to tell Mao the truth. The consequence is a three-year-long Great Famine. Tens of millions of people starved to death, but these are the lucky ones; millions more famished children survived the famine but lived lives worse then death.
Although China today does not have freedom of speech, China does enjoy a very healthy dose of criticisms from the international community. Thus the lack of freedom of speech must not be count as a contributing factor to China's well being.
Here's another Mill's quote:
“If all mankind minus one, were of one opinion, and only one person were of the contrary opinion, mankind would be no more justified in silencing that one person, than he, if he had the power, would be justified in silencing mankind.” ― John Stuart Mill, On Liberty
Free-ish speech is required to have meaningful democracy. It isn't very useful to ask people their opinion if you told it to them.
Democracy is hoped to limit the people's friction with government, and limit how wrong government will be before correcting itself.
Of course very few places have ever gone so far as to try to enforce a heterodoxy and nowhere has a unconditional freedom of speech, so we all live somewhere on a poorly ordered spectrum between those extremes.
Places that have some degree of freedom of speech often have plenty of people willing to publicly look down on places that have less. This can have trade implications.
The desire to have control and influence over others is human nature for most people. Because of this, In order to have a free and just society, you need some sort of framework which resists peoples' desire to control others. A codified freedom of speech such as that implimented in the First Amendment is part of such a framework.
Freedom of speech is particularly important if you want to bring about societial change. Not all societial changes are good, but many are good or even celebrated, and speech and communication are crucial for both building populartiy for such a change and for working out which changes are worth bringing about and which changs aren't.
Allowing exceptions to free speech is troublesome, becuase usutally, when someone wants to restrict it, it's because they believe a certian form of speech to be reprehensible, they'll have a reason for why it's reprhensible, they might feel strongly about their reason, that reason might be widely accepted, but those reasons are not always correct or just.
You might be tempted to assume that people can tell the difference between truly reprehensible things and things that are not, but history shows that to not be the case. There have been several examples over the course of our history where a thing has been widely considered to be rehensible, that today we would consider benign or even positive. Communism, Homosexuality, and Inter-racial marriage are examples of such things. A freedom of speech that allows even reprhensible speech protects against this tendency by taking the decision to allow speech or not away from people.
Free speech allows the unfettered flow of information from one individual to another without self or otherwise imposed censorship. This allows information to more accurately reflect the nuances of the metaphysical struggle of ideas. When one allows for overt censorship of ideas one finds objectionable one is actually attacking the very basis of thought itself. If one is not allowed to express oneself openly about any topic whatsoever one is basically a slave to the overarching ideological system that imposes that restriction.
Information could be censored or suppressed and anyone discussing such hypothetical information may be labeled a dangerous subversive by the media and those in power.
TLDR: If you are censored you are being ideologically oppressed and thought itself is the sufferer.