It's (also?) used for self-checkout: put all of the items in a box, and it tallies up your total automatically, without scanning individual items.

It's one aspect. Uniqlo also uses them for faster ring ups at the counter. They just stack you items over the reader and it gets every tag nice and fast with a final sight check by the cashier that they got everything.

I'm not sure but I think it has a weight systém also to help with catching missed items but this I might be wrong on.