Use dashes to set off or emphasize the content enclosed within them or the content that follows a dash. Dashes place more emphasis on the enclosed content than either parentheses or commas. We also use dashes to set off an appositive phrase that already includes commas.
An appositive is a word or phrase that adds explanatory or clarifying information to the noun that precedes it.
Example 1 - The U.S.S. Constitution became known as “Old Ironsides” during the War of 1812—during which the cannonballs fired from the British H.M.S. Guerriere merely bounced off the sides of the Constitution.
In this case, the phrase that comes after the dash is more important than the independent clause that comes before.
Example 2 - To some of you, my proposals may seem radical—even revolutionary. Here the dash works in conjunction with “even” to emphasize the adjective “revolutionary.”
Example 3- The cousins—Tina, Todd, and Sam—arrived at the party together. Here the dash is not being used for emphasis, but to stand in the place of additional commas that might confuse the reader.
Whereas dashes are used to emphasize content, parentheses are used to downplay content. They place less emphasis on the enclosed content than commas. Use parentheses to set off nonessential material—such as dates, clarifying information, clarifying information, or sources—from a sentence
Example 1 - Muhammad Ali (1942-present), arguably the greatest boxer of all time, claimed he would “float like a butterfly, sting like a bee.”
Example 2 - Denis Johnson’s new novel (which is bound in a luminous red hardback cover) is a worthy addition to the crime fiction genre.
Notice that information enclosed in parentheses has little relevance to the primary idea or meaning of the sentence.