The watch established that people will wear a computer continuously if it is useful enough. The next stage is working out where else on the body sensing belongs.
Rings
Small, unobtrusive, and better than a watch at sleep tracking because nobody minds wearing one to bed. Limited by size – no screen, small battery, few sensors. Good at the passive monitoring job and nothing else.
Earbuds
Already worn for hours, already in a location with good access to blood flow and body temperature. Adding sensors costs little. The obstacle is that people take them out, and a sensor not worn is not sensing.
Patches
Adhesive sensors worn for days, used in medicine for continuous glucose and cardiac monitoring. Moving towards consumer use, constrained by cost and the fact that they are disposable.
Clothing
Sensors woven into fabric, which is the most natural form and the hardest to make durable through washing. Promising in sport and medical settings, far from mainstream.
What the sensing is for
The honest answer is that most of it is not yet for anything specific. Continuous data on heart rate, temperature, movement and sleep is collected because it can be, and the applications that make it useful are still being worked out.
The exceptions are medical – conditions where continuous monitoring changes management – and those are the cases driving real adoption.
The data question
A body worth of continuous measurements is intimate information, and where it goes matters more than what device collected it. Reading the policy before buying is the boring advice that nobody follows and everyone should.