Presumably the GPS does not perform as well without access to cellular towers or Wi-Fi. But this could potentially save a lot of battery life when you’re in an area with poor cell service. Plus, it would let you track your flight process while on an airplane, if you had the maps preloaded.
I can attest this is the case in 8.4, as my photos show from my recent flight. Phone completely in airplane mode, wifi off, all my photos are geotagged. Which is pretty neat as I snapped photos out of the plane and had no clue what I was looking at.