Having a fast, cheap and detailed feedback loop is useful for testing for the bad guys, like in any programming process. For example, in the credit card case,

Having a fast, cheap and detailed feedback loop is useful for testing for the bad guys, like in any programming process. For example, in the credit card case, you can check thousands of cards without needing to find a merchant who will take your thousands of small, frequently-declined transactions. Or you can check the details for cards you haven't fully compromised (maybe the guy selling you stolen bank cards only gives you a portion of the details until you've paid him).