The Em Dash—often now incarnated without spaces—is a hallmark of modern online writing. To some [weasel words] it's become the semicolon of the 21st century, a reckless and overused item of punctuation that signals an author of imprecise and formless prose. These people are wrong, writes Clive Thompson. The Em Dash is awesome.
6) The m-cash evokes the cadence of conversation
As Rachel Holliday Smith — a reporter for The City — notes, the m-dash "gets at the emphasis of how people really talk."
She's right! That's part of what I notice when I'm vocalizing my lines. M-dashes often have a curiously oral cadence. It's why one can use m-dashes in a way that seems to utterly violate rules of grammar yet nonetheless feels perfect.
This terrific essay by Adam O'Fallon Price in The Millions quotes several terrific examples of writers using m-dashes in these odd, conversational ways, such as this passage from Vladimir Nabokov
