(Originally a comment on Chicago poet, Clara Rose Thornton's phasebuke)
use the em dash as an explicit parenthesis - when speaking out loud
something as an aside - that is incorporated into the flow of the
sentence you're writing. The other use is to distinctly separate the
final word/s at the very end of the sentence that a comma doesn't quite
achieve - yet.
five or six years ago i was experimenting a lot with them in my daily
online creative-critical and conversational speculative-discourse
writing, coming up with a theory that there's a basic four beat
punctuation system with which we regulate the flow of our thoughts as
they come out and we write them down on the page with which we are
showing off to the Reader our intellects and imaginations.
use a comma to halt the flow, with a quarter-beat; then you've the
semi-colon a half-beat: with the colon (as British-Hungarian poet George
Szirtes describes it) a 'miniature drum roll' signalling that something
important is about to be announced after it: the longest pause of all.
And with the period being the mark that ends it all when we start anew.
Then the em dash as described - to mark an explicit aside in what is
being said - (the opposite of bracket parenthesis, which is more like
communicating to the Reader privately in a whisper) - and also to make
the end bit of a sentence stand out because a comma doesn't quite do the
job - if ye noo worra mane duk.