One important reason (for me) to switch the phone off, or onto Air Mode - is to save battery. Yes, I could faff around with yet another cable to charge the phone, but there might not be a USB.
If there is a delay, or diversion, or problem on arrival, I want every bit of battery I can. I have seen folks out of phone battery in power outages and they are headless chickens. I also have a power booster battery in my carry-on bag.