[Symbiote]

I noticed socks I bought from Uniqlo had an RFID tag in the label. Maybe that's for the same reason? They know which items haven't been sold.

[__float]

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.

[csydas]

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.