It's true: "what gets measured gets done". Writing down and tracking one's progress is important in order to visualize progress, to see what progress has been made so far. It's hard to know where to go if you don't know where you've been, or where you are right now.
Find out How to Get There
However, there is another half to the measurement equation. There's very little point in journalling things unless all that information can be used to your advantage.
Take for instance, our little toy example above. Here, we're trying to maximize how good we feel, more specifically to some scale we've set (in this case, a 9 out of 10). We also happen to be measuring our heart rate, hours slept and miles run per week, along with how many calories we've been eating as, say, a recovery snack (post run).
It would be great to find out how we can 'get there' ie. get a 9 out of 10 on the mood scale. The answer appears simple: sleep for 8.5 hours, have a resting heart rate of 75, run 5 miles a week, and eat about 250 calories for our post workout recovery. That's how we get to the 9 out of 10. You can turn this around and tackle it from any angle. How about "how do I run 5 miles?". The solution is of course, unique to what you do, and what you log (honesty helps, of course).