The value rarely shows up where you expected it to.

I kept a rough log of my sleep and mood for about a year with no specific goal. Mostly forgot about it. Then I had a weirdly bad few months and went back to look — turns out there was a pretty clear pattern I would've never noticed in the moment.

Maybe the framing of "was it worth it" is the wrong question. It's less like an investment with a return and more like keeping receipts. Useless 99% of the time, then suddenly you really need one.