I've tried a couple wearables like the Apple Watch and they all sucked.
I know a woman who wore a device on her arm that displayed various health metrics that she used to track if she was active enough throughout the day to lose weight. If the device told her she needed to be more active to counteract her food intake for that day, she would take a walk, do yoga, or exercise in some other way. So the device helped her keep on top of her weight loss so she could always be burning more than she was taking in. I literally saw this woman lose about 50 lbs. in four months this way.
It seems that the Apple Watch also provides similar metrics for monitoring one's health and fitness levels, so in this particular case I'd say it COULD be useful if people actually use it for that. But most probably won't so it will end up in the useless gadget pile.