I use QS tools to define where some degree of normality is, so my simple monkey brain doesn't lie to me and I can see when I cross that line :).
Left to my own devices I'll end up eating out for all 1-2 meals a day, only move from the bed to my car to my desk, forget what sunlight looks like, pull 12 hour days and 4 hour nights of sleep (or stumble into a 48 hour cycle), IM only my closest friend every few months, get suicidal and quit another job because nothing matters and the world is cruel.
For me at least, quantified self is not about trying to treat myself like a machine, it's about trying to remember not to.
I find this sort of thing especially helpful when trying new medications/treatments. It's nice to have a chart that I can look over to help give me a better idea about what's going on with my body. "Oh, ever since I started this new medication, I get headaches when I don't drink at least 4L of water."
There's no way that I'd be able to keep track of my day-to-day symptoms without writing them down; the days often blur together.
Oh, and doctors love it, too.