Save your ass
There's one somewhat overlooked feature of the Health app that's worth knowing, since it could very well save your life: the Medical ID.
In the app, tap the tab in the bottom-right corner and fill out some basic vitals info (blood type, allergies, doctor/emergency contact info, etc.) Basically, this serves as a digital version of a medical bracelet -- readily accessible by paramedics and other well-meaning persons, even if your iPhone is locked (it pops up when you tap the Emergency button on the lock screen). You never know, it could come in handy in any number of precarious situations.