1. To set off material for emphasis. Think of dashes as the opposite of parentheses. Where parentheses indicate that the reader should put less emphasis on the enclosed material, dashes indicate that the reader should pay more attention to the material between the dashes.
2. Use dashes to set off an idea or an appositive within a sentence. A dash (—) is a punctuation mark used to set off an idea within a sentence and may be used alone or in pairs. Dashes interrupt a thought in a more dramatic way than a phrase enclosed in commas, but less theatrically than parentheses.