Some GoPro cameras have user-replaceable batteries, and some don’t. If you need a camera for camping or several days away from home, you’re going to want the former. If you buy a GoPro camera with a swappable battery, you can keep extras with you, so you’ll never miss filming the perfect moment. On the other hand, if you’ll only be using your camera for an hour or two at a time, you can save money by getting a model without a user-replaceable battery.
And sure, those options could include your waterproof and crack-resistant smartphone, but keep in mind that the best action camera is designed for this kind of shooting. Plus, just because your phone is tougher, it doesn't mean you should risk losing or damaging the thing that keeps you connected to work, family and friends. It's also way easier to mount one of these to your body, car, bike or anything else than your phone.