For the most part it is to check to see if you have something that you didn't pay for. On the other hand, at least at Costco they are also checking to see that the checkout people didn't screw up. I don't know how many times they asked me "did you get 2 of this item? and 2 of this item?" 90% of the time I did, but the once or twice that they caught the error before I got out the door, and they refunded me the money right there. I was actually glad, because then I didn't have to go back and explain that I did not buy multiples of this item.
