The m-dash is used to set off anything parenthetical from the main sentence. This could be an apposition, a summary, or even a phrase that stands in no relation to the main sentence at all, except by vague association.
That's why it is a bit of a lazy mark: you could use it anywhere, and it gives the reader little information about the relation between the phrase and the sentence. Many style guides advise that it be used sparingly. I use it to mark off a phrase as I might say it in speech after having eliminated all other possible marks. I have a tendency to make too long sentences, because I like appending additional clauses; as several long clauses set off by commas can make a sentence unwieldy, I would use a dash after some point, when I ran out of commas; a dash is a strong mark that may divide a sentence in two more or less separate parts. This aversion to long sentences might be what motivated Gould's dash. While acceptable, the dash could be replaced with a semicolon or comma.
A dash is particularly appropriate to mark a sudden change, as they often occur in unfinished sentences:
Good Evening, Sir. May I offer
you—yes, the Lady Helen has
arrived. Thank you, Sir.
The servant's addressee apparently interrupted him with a question. This is an example of an anacoluthon: a sentence whose syntax is broken off and left unfinished, whereupon a new syntactical construction begins. This is perfectly normal in dialogue, and it may be used as a figure of speech in other writing as well.
You and this Paris—but what did you see in him?
Here the natural flow of speech is emulated, though there is no interruption by another person.
The Spartans, the Athenians, the Mycenaeans, the champions Ajax and Achilles—how is a single city to defeat such formidable adversaries?
The change from enumeration to question naturally breaks the syntax. A dash is often used where a sentence continues after an enumeration.
The Trojan War was probably caused by a conflict between the mercantile interests of Troy and those of the Greek city-states, both powers vying for control of the important trade route from the Black Sea into the Mediterranean—even so, it is not inconceivable that personal feuds should have precipitated the war.
A semicolon could have been used instead; but the dash is supposed to mark the "sudden" change from mundane geopolitics to the more striking factor of romantic rivalry.
Paris abducted Helen with the help of Aphrodite—for how else could he have infatuated the King's wife?—and thereby started the Trojan War.
The dashes could be replaced with round brackets. The writer chose dashes because he felt that the parenthesis, comprising an independent sentence, justified the heaviest marks of separation available.
It was only to be expected that Troy should fall, because the Gods and heroes on the Greek side—Achilles, Ajax, Athena, Hera, even Zeus, and many others—were among the most powerful and the most infuriated.
Here dashes are used instead of brackets because the apposition is a bit long, and the use of brackets is generally not preferred with longer phrases. Using brackets more than once a page in serious writing is generally not recommended, because it can be ugly, difficult to parse, or both.
