Earlier quoted context omitted.
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…
It can also help the customer - it stops you scanning twice (and paying twice) for a single item.
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".