Need to give a credit card number for a free trial, or want to test some code that validates against credit card numbers? That's where GetCreditCardNumbers comes in. It creates "real" numbers you can use so you don't have to give up your actual information.
The site doesn't provide any other details beyond the 16-digit number; there's no expiry data or three-digit CVV number. That means that while you can potentially use it to sign up for "free trial" sites, it won't work if the site actually validates the information, rather than merely storing it. Regardless, it's definitely a useful resource if you want to test credit card acceptance in your code.