A couple times I've started to have my heart rate go out of control while not working out, and it's been a sign of not eating enough recently in my case. While not medical-device-level accurate, having a heart rate monitor on hand has been pretty helpful for monitoring that issue and my recovery from it.

I also like sleep tracking, because I don't get enough sleep, and my Fitbit helps keep track of my failure.

That being said, I bought my Fitbit as a smartwatch, not a health tracker. It was the only smartwatch that supported Windows Mobile.