LLMs also generally don't put spaces around em dashes — but a lot of human writers do.

I think you're thinking of british-style "en-dashes" – which is often used for something that could have been separated by brackets but do have a space either side – rather than "em" dashes. They can also be used in a similar place as a colon – that is to separate two parts of a single sentence.

British users regularly use that sort of construct with "-" hyphens, simply because they're pretty much the same and a whole lot easier to type on a keyboard.