You are probably aware of the Health app that Apple added to the iPhone in iOS 8. Since its inclusion it has been tracking your steps using your iPhone's accelerometer and M7 or M8 chip and storing that information securely in the App. The app however can track and store a lot more — a whole range of exercise, nutrition and medical information can be stored here either by manually adding values or via apps (many of which may be connected to other devices). I've been adding my approximate periods of sleep, for example, by hand every morning. I've recently started adding my caffeine intake and I'm planning on adding my weight soon (I don't currently own working scales).
It's a little bit addictive and it's interesting. Seeing these numbers derived from your activity gives you an interesting new perspective and it is qualitatively more useful than the vague notions of your, say, excercise patterns than you previously had. It won't make you healthy but it allows you to compete with yourself and it allows you to see exactly what that walk to the store means compared with driving there. It will be interesting to see if there is any correlation between my caffeine intake and my sleeping patterns.
The Apple Watch will add some new metrics (improved pedometry and heart rate) as well as having its own health and fitness features. I'm hoping this will not only expand the data I'm able to observe but also provide additional inspiration to improve my lifestyle. I'm not going to start running marathons, but there are innumerable simpler ways in which I can dedicate a little of my free time to staying active.