Functions of the Dash
The dash is one of the most effective punctuation marks of all. It can halt readers in their tracks—it makes them pay attention—as they read through your words of wisdom. Basically, the dash creates the biggest of the "punctuational pauses," the smallest being the comma; next, the parentheses; and finally, the dash. Though the dash does have other uses, showing up primarily in creative writing, it can help expository writers in five situations.
1. Explanatory or Defining Phrases
Set off explanatory or defining phrases with a dash.
Of the three punctuation marks producing a pause—the comma, the dash, the parentheses—the dash produces the most abrupt pause of all.
The dash can halt readers in their tracks—it makes them pay attention—as they read through your words of wisdom.
Pauses in sentences—explanatory phrases, defining phrases, parenthetical material, and introductory defining phrase—prompt many writers to use the dash.