>What does it buy you?
Well, for one thing, it's not a transactional question of what it "buys". It's a matter of principle and defense against future repression or manipulation by politicians on a power trip.
For example, given Trump's current and blatant attempts to crush free expression against his own policies and bullshit, or even those who constantly insult and criticize him (whining about it like a little kid actually) imagine how much easier he'd have had it if there were no U.S 1st amendment to use against him.
There's an example of its value. It's just one of many.
If you think being able to protect free expression and the ability to speak out freely against power and its abuse is anachronistic, then I don't know what else to say except that you're a naive or dishonest fool, and possibly part of the very problem in places where péople just don't seem to care that under pretext X or Y, they can be stifled at any time.
Yes, the social media companies produce, or facilitate the production of, vast amounts of misinformation, disinformation and even hate speech, but guess what? All that shit gets produced en masse anyhow by repressive authoritarian regimes with narratives to construct and agendas to maintain. Free speech certainly isn't at fault for its existence, given that such things have existed since there's been propaganda or a perceived need for it.
At least, in a place like the U.S, where free speech remains protected (for now at least), any misinformation, disinformation or whatever speech by those in power or outside of it who create it, can be countered by others trying to speak more truthfully.
Try doing the same against misinformation and disinformation by government in Russia, or many other countries where "anachronistic" free speech is curtailed right to hell.
In essence, when governments can legally censor speech they decide is misinformation, disinformation or "hate speech", they can create all sorts of um, interesting, rubrics for deciding what fits under these labels, and then oops, by coincidence it can be anything that goes against their agendas. Going back to the Trump example, just pause for a moment to think about all the uncomfortable facts and opinions he loves to label as "fake news" or "misinformation" or even as hate speech. Now imagine him having the legal authority to sweep them away.
Nothing in any state guarantees against a future leadership with similar authoritarian proclivities from forming to use anti-free speech laws in similar ways.
There, my good faith response to your completely absurd line of rhetorical questioning.