Or to buzz every 30 seconds (or however long you're going) for HITT workouts. I've tried using MP3s with beeps put in every 30 seconds, but the headphones fall out/off/get knocked around when I'm sprinting full-bore. MP3s also have the disadvantage of forcing you to start right on a beep, so if you get interrupted for a second or two it throws off your workout. A HRM could start whenever you started.