Many instances. You are out sightseeing, your phone battery dies. You summon an Uber on the watch. However you can’t find the driver so you need to talk with him. You call on your watch but you can’t hear anything over the street noise. Driver cancels, you lose $10 for the time, and you still have no ride.
Perhaps it’s a cultural thing, but here in London most people would do anything rather than making a call on their watches in public lol