Dashes cut a fine figure – a fine, swashbuckling figure.
When they enclose a phrase – even a modest phrase – they do so with élan.
Beware the dash – it is addictive.
Dashes can replace a colon to conclude an introductory thought and introduce an amplification of that thought.
Dashes can be used like a pair of parentheses to enclose a sentence element which is not part of the main structure, but additional information. A pair of commas or a pair of parentheses can also be used for this purpose, but dashes give more emphasis.
There is a temptation to use the dash frequently because it is a multi-purpose piece of punctuation; if you aren’t sure whether to use a comma or a colon, it’s tempting to use a dash. However, anything more than a rare dash makes prose seem excitable rather than calm, emotional rather than rational. Academic writing uses them very sparingly.