Aside from demonstrating just how much of a geek I am when I wrote 'Know How You Measure', I opened the door for a few people that know me fairly well to ask me why I spend 'so much time' measuring things like... mileage. In the context of a gearhead car geek, it's simple.
You can tell when things go wrong, as well as tell if what you do is an actual improvement. It's really that simple.
As Peter Drucker wrote, "You can't improve what you can't measure." You can think you improved something - like, for example, the air ram or, as they call it these days, 'cold air intake'. It's been proven (there's a video in that link) that it pretty much doesn't work (in a stationary vehicle on a dyno - 'know how you measure', right?). We all have theories on how things can improve, but in the end we have to take into account whether or not something is an actual improvement. How do you do that?
You establish a baseline. It's where everything starts. True gearheads do it. Engineers do it. Doctors do it (why do you think they do all those tests and x-rays?). Everyone who wants an improved result has to at first have a result to improve upon.
But it's not just improvement. It's also about catching failures. In the example I gave in 'Know How You Measure', the main reason I track mileage is not because of improvements but because mileage is a good indicator of whether something is wrong, particularly in a complex system. It's an integral part of maintenance as well as development.
In summary, it's about measuring quality, where quality is easily defined for a man-made system: Works as designed.