Maybe you forgot your phone somewhere, or the battery on the phone died.I honestly don't understand the point of standalone GPS on the Apple Watch. Why not just use the phone's GPS? I can't see any situation where you'd have your watch but not your phone with you.
I wonder if the swimming workout uses the GPS if you're swimming outdoors. If so, you wouldn't want to bring a non-water resistant iPhone with you.
I don't see what anyone would object to having the GPS on the watch, other than added cost.