When I drive up, I just say "online order for function_seven" and they say, "cool, we'll have it at the window".
The staff taking orders from the speaker are often doing 2 things at once. They're also taking money from another car or prepping orders as well. They don't catch every nuance, and screw it up half the time. Not their fault! They're not being allowed to focus on one thing.
The app is like skipping that unreliable middleman and putting my order directly into their system.