> - dash: compounds word-parts (Sarbanes-Oxley)
That's a hyphen, and (to be pedantic) style guides usually do not consider it a dash.
There's also the 2-em dash in some style guides for redacting words, and since care needs to be taken to avoid a line wrap between the em dashes, Unicode has a character for it: ⸺
Finally,
> - em dash: adjoining thought (Hey, so — <declaration>)
This use is sometimes discouraged by style guides, and in many cases, it can be replaced with a colon, comma, or semicolon.
In general, even before AI, the em dash is often considered overused.[0] My guess is that the em dash is perceived as sounding educated, so novice writers who learn about it end up using it recklessly, and LLMs could've picked up this habit from them.
[0]: https://slate.com/human-interest/2011/05/em-dashes-why-write...
The advantage of the em dash is that it can replace pretty much any other punctuation mark and work well enough. Quite often I'll be trying to remember whether to use a semicolon or a colon to connect two parts of a sentence, and then I decided I can't be bothered and just use a dash instead - it'll be fine.
I think this is the real reason for its (pre-LLM) ubiquity - it's a really flexible, versatile bit of punctuation, whereas most of its siblings have a more specific meaning or purpose.
The best advice is often: Use a dot and start a new sentence. The best writers use very short sentences in a row and simple words. They flow easily and do not break reading. This works particularly well in science and for difficult topics.
Most of the best writers today, and aesthetically the baseline expectation. But I personally like the older em dash and semicolon laden styles that used a greater diversity of forms to weave thoughts together.
A stream of short sentences often feels lazy, reductive, and tedious. But it is more forgiving--you can't fail if you don't try. And it inflates word count, similar to verbose Java vs compact Perl.