Let me repeat: They are ONLY able to use the barometer if they compare it to the barometer reading at the airport they're about to land at.

If they don't, their barometer is useless. Atmospheric pressures changes constantly. Can you check somewhere with your bicycle barometer to check with an already known and absolute height? No, of course you can't. Hence your frigging barometer is not accurate.

The "check" with the airport is done so they know what the pressure is at that precise location. They know exactly what absolute height that airport barometer is at, and then it's simply a matter of subtraction/addition to get your height. Does your bicycle computer comunicate with a VERY nearby known-altitude barometer? Of course not.