Question

When working with additional information in a sentence, when should dashes be used instead of commas or parentheses?

Answer 1

Em dashes are used to put aside fairly important information, but in a way that will attract the reader's attention.
Examples:
You are the kitten—the poorest kitten—that I've ever seen!
The Ames Sentinel—dated May 1, 2013—arrived in June. (quoted from GrammarBook.com)
Joe—and his trusty mutt—was always welcome (also from grammarbook.com)