I guess because people want to track some things about their health, and people provide good pieces of software with a simple UI to do it, and this is more useful that, say, writing it down in a notebook, or in a text file or notes app.

I guess also people feel that corporations _shouldn't_ be allowed to do bad things with it.

Sadly, we already know with experience in the last 20 years, that many people don't care about what information they give to large corporations.

However, I do see more and more people increasingly concerned about their data. They are still mainly people involved in tech or related disciplines, but this is how things start.