A smartwatch is the perfect accessory to a device like this, which is too large/unwieldy to use at the gym, or pull out for things like map directions, public transit arrival times, email notifications, etc. None of these are a problem with smaller phones, like the 6 Plus or Galaxy Note.
The Apple Watch is a poor solution for this, because, like the 2001 iPod, it requires a Mac device to work. And even if it didn't, like Android Wear devices, it must be tethered to a smartphone for most of its functions, as opposed to functioning as a standalone device.
I understand Apple, Google, etc. don't want to cannibalize device sales, but it seems ridiculous to me that a V1.0 smartwatch costing $300+ can't even support music playback on local storage, or have a built-in GPS and maps that work offline. If I'm on the go, I'd like to limit the number of expensive devices I'm carrying with me where possible. I think the same goes for most people, otherwise people would still be carrying their flip phones and digital cameras around. Yes, of course there are hobbyists who need their DSLR with them all the time, but the majority of people are happy with a solution that's good enough and convenient enough.
I'm not even thinking about more advanced features like a built in video camera that, with the appropriate strap, could act as a body cam or a helmet cam. I'm sure future versions will offer such functionality, but V1.0 functionality is already so limited, I simply can't muster any enthusiasm for this class of devices yet.