With sixteen digits each, the unique numbers that are found on credit and debit cards issued by providers like Visa, MasterCard, and American Express cover a huge range of possibilities. While there are quadrillions of permutations in general, the various restrictions placed on these numbers mean that quite a few less will be valid in practice.
Even so, the variation that remains is a cornerstone of security for such cards and the accounts associated with them, making it fairly difficult for criminals to simply generate new, valid numbers at will.
While that is obviously advantageous in general, it can also pose some difficulties to those with only the best of intentions. Many software developers, for example, have run into the issue of needing to test a new system before putting it into production. In order to test credit card number parsing, handling, and storage, having access to valid-seeming digits would seem to be a key requirement.
Fortunately, this turns out to be relatively easy to see to. Both the providers themselves and others offer up numbers that can be used freely by software developers and others in order to make sure that their systems will be able to handle real credit and debit cards once they go online.
Using a test credit card number, on the other hand, will often vary a bit from what is to be expected when it comes to the real thing. For one thing, the CVV security number that is attached to most actual credit and debit cards will often be missing, with any three- or four-digit combination being accepted by a test system. The same will often go for other identifying features like the billing zip code, address, and even the name associated with a particular account.
While it could be useful, in many cases, for details of these kinds to line up a little more closely with reality, developers will normally be able to set their own restrictions in their test cases to overcome this small hurdle. What will normally matter the most is simply that credit card numbers of these kinds make it much more likely that a new system will not fail in unexpected ways when it finally goes online.