One of my favourite comedians, Bill Burr, ranted brilliantly about the wristbands you can put on your kids so if they wander off at the park you can call them back or go find them. He saw it mentioned so casually in some other advert unrelated to the wrist thing and he was like "..wtf was that thing the mom used?!"
Joking aside, one application could be GPS for people that go climbing or exploring, or for people living in earthquake-prone regions to find them in rubble. But you can do that stuff without going subcutaneous.