Sit down. I'm about to take you on a trip.
Metal Gear Solid V: The Phantom Pain has, in addition to its Giant Robot and Secret Advanced Tech elements of science fiction, one that involve language via sound-activated parasites. Speak that tongue? Die horribly. The scheme was to modify said parasites to target Anglophones, and thereby disrupt power in the world by overthrowing the dominant tongue (and the ideas it best spreads).
Daddy Warpig has, at a site about a tabletop RPG called TORG, gave the most useful definition of magic yet: "Magic involves people causing effects by manipulating symbols that represent the desired effect."
It gets better. He expands on it immediately after that:
Let’s expand. A symbol can be anything: a word, a picture, a gesture, or an activity. An effect is a specific desired end: cause injury, cure an illness, bring good fortune. In magic, people manipulate symbols that represent the effect and by doing so, cause the effect to actually happen.
(Bold is original. Underlining is mine. Pay attention to the emphasized words.)
Words. Language is made up of words. Words are symbols used by Mankind to communicate what he knows about his world, past and present, to his fellow man. Speech, language, is therefore powerful because it is the primary method of symbolic manipulation used to create effects in the world, including upon his fellow man.
It is known that most magical traditions have an origin in religion, but with a turn away from service and towards mastery (in the sense of domination). It is common to depict a magician as being charismatic, beguiling, or enchanting in his speech because their practice of magic has them routinely commanding things with a forceful, willful presence. Summoning demons, calling forth angels, and other supernatural staples have a booming, commanding voice involved. Casting spells and performing rituals rarely lack the use of language as a key component.
From there we see why Rhetoric--the art of persuasion--is often hated as "sorcery", as it uses language to manipulate emotions in order to get the audience to do as the speaker wishes. Confidence artists, politicians, salesmen, etc. are all put down for their skills with language, while those who condemn them are praised for theirs and put in authority. It is no accident that many a religion condemns sorcery, and not for cynical reasons necessarily (as any father of a willful child, or husband to a foolish wife, comprehends).
So, Mankind--in a very real way--is an inherently magical species. How do you control such a species? We have ample evidence of many methods, but none work so well as control of thought. No means of thought control--of mind control--work so well as control over the symbols themselves. If you corrupt a word or phrase so that it loses its meaning, it also loses its power; this is the truth powering parables such as The Boy Who Cried Wolf. By destroying the power of a word, you destroy what it represents and thus kill the idea behind it. Thus do you kill things immaterial, but real nonetheless: Ideas.
That's what the parasites in MSGV:TPP symbolize (there's that concept again): the power to control thought via control over communication thereof via language. If you can choke out all symbols (words) you don't want spread, you can choke out ideas by killing propagation of them. This is how and why Rhetoric gets its bad reputation, because this is one of its applications. "Thought-terminating cliches", and similar Rhetorical weapons, are the real world versions of those parasites and they have the same goal: to terminate propagation of targeted ideas via degradation of the words and phrases used to spread them, silencing them into death and extinction. It's memetic warfare taken to its logical conclusion.
And now you know why Free Speech matters, and why you need to know what Madison Avenue does and how it does it. (And if you don't understand what Madison Avenue symbolizes, then you've already experienced the consequence of language destruction.)