I think these exist entirely to squeeze labor costs, but I'm not sure that's a bad thing.
First, allowing grocery stores to operate with smaller staffs means we get more grocery stores. The small, cramped grocery store that recently opened within walking distance of my apartment probably wouldn't be able to operate if it had to provide not just pay, but parking and work space for another dozen full-time employees. That's a food desert in a dense urban core that doesn't exist anymore and a smaller, but still substantial, group of available jobs that otherwise wouldn't be there.
Second, if I were to approach you and tell you I had two plans for a machine. Both are about evenly efficient, but one requires two people to stand at the machine doing boring, repetitive, mindless drudge work for hours on end. I, for one, definitely prefer the one that doesn't require human suffering to operate. Automated checkouts eliminate jobs, but specifically they eliminate shitty jobs that we should probably feel bad about asking anyone to do in the first place.