Removing cashiers and automating checkouts just frees up the staff to focus on other in-store work. But the biggest benefit is higher throughput for checkouts. Self service checkouts or automated checkouts free up space for more such counters.
The supermarkets are functioning fine which leads me to think that there isn't a whole lot of other in-store work waiting to be done if only more staff were free to do it. Automating the checkouts just reduces the levels of staff that you need.