In the book “Eats, Shoots & Leaves: The Zero Tolerance Approach to Punctuation,” plucky Lynne Truss describes punctuation as “a courtesy designed to help readers to understand a story without stumbling.”
With this in mind, I would name the em dash as the most chivalrous punctuation mark of all time.
As I often explain to punctuation newbies, em dashes (or long dashes) are used to indicate a pronounced interruption or break in thought. They should be used sparingly and only when another punctuation mark (such as a comma or colon) will not suffice. For example:
The best use of an em dash I have ever seen was in an issue of my alumni association newsletter. The article was introducing the newest longhorn selected to serve as Bevo, the University of Texas mascot. A brief family history of the animal included this sentence: “Bevo’s father was killed when he was struck by lighting—his hooves and horns were blown right off—leaving Bevo an orphan.”