This is so when a hacker gets into a database w/ a ton of credit card numbers they are unable to make transactions with just the numbers.
In accordance with the card issuers agreement the CVV# cannot be stored after the transaction has been processed, and a merchant cannot process a transaction w/out the CVV#. This is why a merchant will always ask for your CVV# even if they have your credit card information on file.
Edit: changed wording from store to merchant to prevent confusion.
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