This will be useful if you’re trying to accurately log and track your exercise. For example, if you were to start running, the watch might not be able to tell if you’re running on the treadmill or if you’re doing laps around the park. It can also be good for energy savings, where if you’re running indoors, then maybe there is no need to activate the GNSS receiver.
There have been some criticisms about fitness trackers and smartwatches where tracking the user’s activity on a stationary device, like a bike at the gym or the treadmill, isn’t as accurate compared to an outdoor activity. It will be interesting to see if Apple does indeed have plans to implement such a system to the Apple Watch, but unfortunately with this being a patent, it’s hard to tell if it will ever see the light of day.