Real world answer: People who run long distances and don't want to carry their (increasingly larger) smartphones with them, but must have a method of communication with them for safety reasons.
For example, I run multiple marathons a year and am training year round. I go on long runs all the time. I also live in the middle of a very large non-pedestrian-focused city with a lot of drivers who are generally oblivious to pedestrians, and have had several close calls over the years. Additionally, I'm an asthmatic and while problems are rare–there's always the threat of something bad happening. I have to have a way to call in case of an emergency. For people like me having a watch that can do emergency calls (or allow you to receive messages while you're on a 3+ hour run just in case someone else is having an emergency worth cutting your run short for) without having to carry a large device with you is very appealing.
Edit: Clearly this isn't a need that isn't covered by a phone. It is, however, a substantial quality of life improvement for some.