Coming Soon, Smartphone-Based Banking
An anonymous reader writes "Banks will be offering a new service at the end of the year that will let customers take a photo of a paper check and have it be deposited in their bank accounts, making the smartphone one step closer to an ATM."
"he scans it and gives it back to me"
In other words, the check was a pointless part of the process. He could as easily entered the numbers from the front of your debit card. Or you could have used a different debit card for the same account (exactly the effect of scanning the numbers on your check).
You'll notice, though, that it's the check that was the insecure means of dumping money from your account. It didn't require a PIN the way your debit card did. That's why your debit card number isn't just a plastic version of the numbers on your check.
Paper checks are essentially useless, except in situations where there are no electronic devices around to mediate the transaction. Your example, unfortunately, didn't exemplify that so much as it exemplified the confusion due to the amorphous technical nature of the banking system.