It will not be news to Irreal partisans that we here at Irreal are enamored of em-dashes. Some say that they are seldom used but I love em-dashes and probably tend to overuse them. Back in the dark ages before ubiquitous personal computers there were typewriters and they had only one type of dash: the hyphen. There was a convention of using a double hyphen for an em-dash but most folks who weren’t professional writers or typesetters had never heard the term “em-dash”.
Until I learned TeX, I wasn’t aware that there are, in fact, three types of dashes: hyphen, en-dash, and m-dash. The TL;DR is that you use a hyphen as a break within words at the end of line, or to connect compound words; the en-dash to separate numerical ranges; and em-dash as a clause separator, sort of like a comma.
The rules are, of course, nuanced and the above doesn’t capture them precisely. For those of you who want to better understand which to use and when, Mister Punctual has a post on the use of hyphens, en-dashes, and em-dashes. Now that reasonable typesetting software is available to everyone, it behooves us all to learn the differences between the various dashes and when to use them.