One of the functions of dashes and commas is to mark an interruption from the rest of a sentence. These punctuation marks can be used in pairs (when they are in the middle of a sentence) or alone (mainly when they are at the end of the sentence) and their main difference is that while commas are used to mark off a weak interruption, dashes mark a strong interruption.
Examples:
He loved me — that's what he had said— but he left abruptly and never came back.
Mark wanted a family — or so he believed.
I liked your company, of course, but I had to go home.
