The only "killer" feature I could see a smartwatch offering is way down the line when smart cars are around i.e. being able to lock/unlock your car from your wrist, locate it in a carpark easily with your wrist, get notifications if it's being stolen direct to your wrist or, being able to start/stop the ignition from your wrist, etc.
That would be convenient. No more keys and you wouldn't have to pull out your phone to carry out these basic operations. Further down the line, I can see something like being able to tap your wrist and your self-driving car starts itself, unparks itself and comes to your location being a super use for it.
But these are quite a way off and even then the cars with these technologies will be far too expensive for the average person currently eyeing up a smartwatch.