It’s a security feature sold to the stores against a possible but rare attack - barcode swapping.
It even occurs with human checkers but it’s quite rare (barcodes are fixed now, or the stickers destroy themselves if you try to remove them like at Goodwill).
The attack apparently would have you swap a barcode but still on camera scan it which I guess theoretically makes it harder to prosecute but I kinda doubt it.
It also helps catch misscans which is probably much much more common (someone scans an item, they thought it scanned, they put it in bagging area).
A pop-up could more easily do that without being annoying. "You already scanned Miracle Food. How many total are you buying? Just one / enter a number / I'll scan each one manually"
More likely it's for absent-minded customers who don't realize the item didn't scan. If there's a sudden weight without a new barcode scanned, the error message on the machines at the store I use say something like "unexpected weight, please remove item and scan again".
In the early days of self checkout I remember being behind a woman who could not figure out how to scan things. She was basically presenting the items to the scanner like it was a photo shoot while getting frustrated that the machine only sometimes recognized what she was presenting to it.
It even occurs with human checkers but it’s quite rare (barcodes are fixed now, or the stickers destroy themselves if you try to remove them like at Goodwill).
The attack apparently would have you swap a barcode but still on camera scan it which I guess theoretically makes it harder to prosecute but I kinda doubt it.
It also helps catch misscans which is probably much much more common (someone scans an item, they thought it scanned, they put it in bagging area).