or,1 "Why people who think that long words are inherently bad and should never be used—because they might not make sense to other people—are wrong."
A common piece of writing advice is to "avoid using long words." According to this line of thought, good communication expresses thoughts completely in simple words, instead of showing off the author's vocabulary—and those who use big words flatter their egos at the expense of getting their point across.
When obscure words are used, fewer people understand them. But this can be for one of two reasons: because they're being used by the author to sound more intelligent (which is bad), or because they serve as efficient references to obscure abstractions (which can be good). Some things are complicated, and require precision: in such cases, sesquipedalian communication is indeed effective.
Look at any academic discussion of science, or at the debates of engineering professionals: no layman has any hope of comprehending the strange words flying around. But this makes sense, because jargon serves to compress ideas which would otherwise take a long time to explain. A programmer can spend paragraphs describing how fast their sorting program runs, or can say "in
O(n log n) time" and be done with it. Nobody without any computer science knowledge can tell what the latter means, but that's usually okay—whoever they're talking to should know, or can ask them what they mean.
But often big words are used to puff simple ideas up, so that they seem more impressive than they are. This is how most people—including me—learned to write: they're given a shallow topic to write a long essay on, and after writing down everything they know, they pull out the thesaurus to meet the requirements by restating their ideas in all sorts of different ways. It's most common in school, but these bad habits trickle into the real world too—and are what the common advice tries to warn against.
Good usage of weird words compresses ideas; bad usage obscures them.
Complex words in a proper setting correctly signal to observers that they are not knowledgeable enough to follow along—and this is fine, because it's true. The reason such language is used is to compress communication with those who do understand, though; confusing normies is a side effect!
But out of that comes danger. Just as vulnerable prey mimic the decorations of their more dangerous cousins to avoid getting eaten, people who want their ideas to be accepted without argument will also use complicated words in the hope that you, uncomprehending, surrender to them (just as you would yield to someone who's above your level). This strategy, an insidious version of the appeal to authority, is called "Eulering"2.
How can you tell one type of obscure communication from another? By trying to understand, and asking clarifying questions. Anyone who understands the big words they use—and means what they say with them—will be able to uncompress their speech: to give you a longer and more familiarly-worded explanation. When a charlatan or poser is using big words, in contrast, asking questions will force them to either (1) backtrack on what they said, (2) give circular definitions, or (3) contemptuously deny your request, implying or saying that you're simply too far below their level to understand.
Unfortunately, it can be quite hard to judge someone if you cannot interact with them. If someone leaves an incomprehensible Reddit comment, how can you tell if (1) you're uneducated in the area, (2) they're crazy, (3) they're deliberately messing with you, or (4) they're correct, but just a bad writer? Sometimes you can't; you just have to rely on your gut, or be content with uncertainty and move on.
Long words are not inherently bad; in proper usage, they serve to compress, rather than to obscure. But since compressed language is often hard for the layperson to understand, those who seek to dazzle you into submission will sprinkle their language with long words to obscure their arguments, hoping to be accepted on faith and authority rather than from a place of mutual understanding.