Sometimes you have some information which needs to be added to a sentence, and that little bit of information is direly – no, deathly – important to that sentence; if it’s a matter of life or death (figuratively, of course), you’ll want the reader to pay attention to that information. Or perhaps you’ve been ranting on about one thing and then suddenly change tack and rant on about something different. In either case, this is where you want to use a dash. It’s a strong punctuation mark, and it can be compared to the softer parentheses (just as a period compares to a semicolon).
You can use a dash whenever you need to wake your reader up and let them know that the focus is changing.