Sure it’s not perfect. Just like the complaints about self checkout lines at grocery stores.
But there are a LOT of customers who will be pulling out that phone anyway while idle in line - nice if the appropriate app just shows up, and with a few taps is done-and-paid with the order well before the overpaid underperforming clerk is available.
Argue all you like, fact is such apps already exist, and people are already using them to avoid dealing with clerks. All the clerk does is enter the information you can/will with the app; why pay someone $15/hr when most customers would rather do it themselves?
I would be interested in seeing these... can you point me to a couple of places that have such a setup?