Re: Warning: dumb question incoming
If you learn in a hands-on way, a couple apps that use it are starbucks, target cartwheel, Kroger, and Walgreens. When you go into those apps at the gift card section or reward card section, respectively, it'll prompt you to add the card to passbook.
I really only use the Starbucks app and Target Cartwheel for passbook, but I know in Starbucks you can set up your favorite stores and then when you get close to the store you'll get a notification (even on your lock screen) and if you tap it, it'll pull up your card information to be scanned. Pretty sure it works similarly with the other apps that use it, because that's the point - making it more convenient.
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