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).

It can also help the customer - it stops you scanning twice (and paying twice) for a single item.

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.

I only just learned about the concept of weight checks, I'd assume they weigh whatever you 'present' to have bought and compare that to what you scanned.

If you don't scan it and you put/keep it in your backpack, that's not different from doing the same but going through the cashier checkout right? They already had that problem, so if they keep that as it was but reduce the amount of personnel, that's a net positive?