Why Does Costco Check Your Receipt?

On a cold day at the Timnath Costco, it's miserable to stand near the door, but you have to stand in a line to leave Costco. You're waiting for the friendly people at the exit to check and draw a line through your receipt. Why do they do this? You just have to look up when leaving to see the answer.

The sign says:

Question: "Why is my register receipt reviewed when I leave the warehouse?"

Answer: "To assure that you receive everything you paid for and are not overcharged or undercharged for any item. Also, marking the receipt disallows its reuse."