The Em Dash: Afterthought or Added Emphasis?

An Em Dash may introduce different sentences whose meaning and effect may vary according to context, but is not limited to an afterthought or to add emphasis:

• The em dash is perhaps the most versatile punctuation mark. Depending on the context, the em dash can take the place of commas, parentheses, or colons —in each case to slightly different effect.

(www.thepunctuationguide.com)

An em dash, or long dash, is used:

in pairs, to mark off information or ideas that are not essential to an understanding of the rest of the sentence: - Thousands of children—like the girl in this photograph—have been left homeless.

• My son—where has he gone?—would like to meet you.

to show other kinds of break in a sentence where a comma, semicolon, or colon would be traditionally used:

• One thing’s for sure—he doesn’t want to face the truth.

• Things have changed a lot in the last year—mainly for the better.

Note that there is no space added on either side of an em dash.

Em dashes are especially common in informal writing, such as personal emails or blogs, but it’s best to use them sparingly when you are writing formally.

(en.oxforddictionaries.com)