The local Walmart grocery has lines at the self-checkout during peak hours. Since self checkout can only be done one item at a time, while the cashier can do multiples, there are times when the cashier lines are a better bet than the self checkout.
And if the machine malfunctions (unexpected item in the bagging area - put item in bagging area on a continuous cycle as you move the item on and off the table), the cashier line is a sure bet. Just make sure they don’t rotate a bag away from you while you are picking them up.
There are annoyances and potential snafus with self-checkout, that’s why I only use them if the open checkouts are backed up. I’d hate to self-checkout with a cart full of groceries. That’s a lot of added effort and time, with no payout unless you’re beating a long line elsewhere.