Yep, using pressure to gauge height is how many altimeters work. Then by removing this degree of freedom your phone is able to calculate a GPS location quicker

Yep, using pressure to gauge height is how many altimeters work. Then by removing this degree of freedom your phone is able to calculate a GPS location quicker as it knows you are confined to a particular area as opposed to a volume. I believe it also means you can get a GPS location when locked to one fewer satellites (though it may be slower in this situation).