I meticulously tracked every calorie I ate for several months. Now I know how much of different types of food is how many calories, how many calories I eat per day, and most importantly what my different hunger levels mean in terms of the number of calories ingested. Combined with my weight tracking, which tells me my set weight / weight variance over a day or several days / food intake to gain or lose a certain amount of weight, I can easily control portions to hit my weight goals without thinking about it.
I would imagine more health data would allow me to optimize in this fashion as well. If I can correlate health markers to my lifestyle often and directly, I can make better and more informed choices. Advocating against easy access to health data because some people can misuse it is the same kind of nanny state thinking that says encryption shouldn't be available to the masses because criminal enterprises can use it to hide their activities.