This was inspired by a post in Google+ that was reshared by a person I follow.
The post was titled "Top reasons people stop using wearable technologies", and listed a bar chart with said reasons. At URL.
Most likely, the market for wearables is still starting out, and hasn't yet found its footing. There is also no standard or common/dominant design, such as Windows in PCs, and Android/iOS on mobile phones.
Since I don't have any wearables, then my knowledge is based on actual media exposure (=article titles) of practical benefits. This has boiled down to two very different brands.
• Much more affordable from the outset;
• Its charge can last a week;
• Works as advertised;
• Its health tracking functions can be used to determine better diagnoses related to heart disease and a number of other ailments, where health logging becomes important;
• Can save lives.
Conclusion: Fitbit doesn't do everything, but it does well the things it's meant to do.
Apple Watch mdash;
• Expensive (aka "Worth its price");
• Short battery life (requires recharging each forthnight);
• Creeping featuritis — not enough useful features vs. too many useless features, therefore no direction.
• Water-resistant, but not waterproof.
• Difficult and expensive to fix, if gets broken.
• The advantages are integration with iOS and Siri. Of course, health improvements, too.
• Apple Pay (if you forget your wallet home), but that's only in the U.S.
• Can save lives.
• Can run Windows 95. But that means other operating systems, too...?
Conclusion: Differing functions, no special use-case. Has app ecosystem.
Footnote: Given that wearables log information about a user, they can also be very useful in forensic investigations, such as evidence-gathering or coroner's reports in determining the exact time of death (that is, if the device clock is correct) or another potentially life-harming event that needs to be investigated.
But this also requires, that wearables data must per user reques be stored (encrypted, hashed, and salted) in such a way that it couldn't be tampered with. I speculate, that current implementations might not meet such strict storage regulations, and their data can thus be used on the basis of assuming good faith.
So, at this time, logs of wearables can be used as supporting material when building a case. Whether wearables data can be used in courts of law, is a matter to be resolved by the legal profession.