I can imagine the "will you be using the mobile app?" question to get cheaper food is going to devolve into the Mark of the Beast and someday no one will be able to buy anything without using the apps
I was using the apps before it became the only place to get the deals simply for the fact that it means I don't have to talk to another human being IRL. It also means I don't have to wait, at least at some places like Jack in the Box, since they start making your order when it's placed and not when you show up, like McDonald's does.
Not Mark of the beast shit, but I do see there being a time where the apps will be the only way to order fast food because fast food will have been fully automated at that point. There would be no humans to take your order at all. It would be a machine, and you interface with it through the app.
Part of me believes that would be great, like all the self-checkouts at the Walmart. But another part of me knows that it would be one of the single most annoying/frustrating things to deal with, let alone the fact that all those people would be out of a job.
If you're cool with McDonald's knowing your location (or at least an approximation of it, not sure how much data gets passed through) keep it turned on.
The app has geofencing tech so when you get close enough to the restaurant they'll start making it. I've had mine ready when I get there most of the time. Of course that depends on how busy they are but just fyi