There's all that you mentioned, but I also think a large part of it is that the customer does the time-consuming tasks of placing the order "Um, I'll have... no, lemme make it... aw, just a burger. But no ketchup" and payment "Now, I know I have that nickel in my purse somewhere..."Part of the McDonald's pricing strategy is to drive people to use the app.
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And of course they've automated the food prep as much as it is possible to do now.
I do have the McD's app on my phone, but only because there's one near the R/C site where I fly model airplanes. I actually only use it maybe once a month or so. More in the summer of course.