People will use this different ways. I do 30 minute workouts, not including warm up or cool down. So I set mine to 35 minutes at the start of my warm up. This way it will notify me when to start my cool down, but then I let it continue to run. I don't actually end the workout until I walk away from the machine.
Also, someone might choose to extend their workout for any number of reasons. Maybe they didn't exert themselves as hard as they anticipated, so they want to go longer. Or they're reading/listening to a book and want to finish the chapter. Etc. I would actually be highly annoyed if my watch stopped tracking me while I was still going.