I use a Garmin that measures all sorts of things. Pulse, heart rate variability, sleep (although not hugely well), and it tracks my exercise. If I work hard enough it can determine lactate threshold, estimate VO2 max and so on. And of course it counts steps. It monitors training status to tell me if I'm overdoing it or need to step it up a little. It lets me define workouts, which is handy because I can set up a base level workout so it beeps at me if my heart rate goes out of the defined bounds. It also allows stages of a workout to be defined based on time or distance with targets for speed, heart rate or nothing (the latter is handy if you want something like a 60 second recovery with no particular target).
It's handy to be able to do something like "run with heart rate below 140 for 10 minutes. Then run at a pace of 8:10 - 8:50/mile for three miles. Then recover for 60 seconds. Then run at a pace of 7:40 - 8:10 for three miles. Then recover for half a mile. Then run at a pace of 8:45 - 9:15 for two miles, and finally run with heart rate below 140 for 10 minutes". Personally I don't define anything that complex but if you follow one of the coaching plans you get all sorts of interesting combinations to improve your speed.