I didn't say that Anafi GPS does nothing, I said quiet opposite (and I asked not to get me wrong). Actually, you say almost the same thing in other words that I said, look it up. Except that this is Anafi have to get some data from app and increase (or decrease, or do nothing) the power of its own wifi transmitter depending on what data it receives from app and compares with its own data. I suppose that changing (or not) the transmitting power of controller is only the second step (you actually can control Anafi without it). You also could check your theory using GPS spoofer.
About your last question. Here is why have it: to RTH, for geofence, for flight plan mode, to measure altitude and correct data from IMU' barometric sensor (AMSL to AGL conversion, but I doubt ANAFI actually does it), for NFZ and height limitations in surrounding areas (OK, last thing is about DJI - but Parrot could also implement it).
