> >employees who get taken away from their other duties to help customers deal with the confusing and error prone kiosks.

> I have seen these customers. I do not understand.

One applicable scenario I have observed at my local super market is people for whom English (or the otherwise locally-dominant language) is not their first language. The machine may have a 'clear' error message displaying and speaking "return last scanned item to bagging area" but imagine that you don't understand the language the machine is using very well. At a register with a human, a person can unload groceries on to a belt, hand the cashier some bills that they may or may not entirely understand the value of or swipe a government-provided benefits card, and get back their items and any applicable change without needing a firm grasp of the spoken or written language.