People use credit cards all day, every day, in which most transactions, but certainly not all transactions are done in person, and hence the actual credit card in the consumer's possession is actually used, but the advent of the internet shopping age, has meant that a fairly large number of transactions are done each and every day via the internet, using a given person's credit card information, and while some of these transactions utilize the same credit card again and again, as this credit card has been previously entered and thereupon saved as the default card, there are plenty of internet sites that the consumer is utilizing for the very first time, or desires to use a new credit card, or doesn't have their credit card previously saved, and thereby the customer must enter the pertinent digits of the given credit card by hand, meaning that humans can mistype or mistakenly type in the wrong numbers, and do so, probably by the thousands each and every day. The most incredible thing about this, is that computers are able to instantly recognize the validity of a given credit card within a second or two, which many people take for granted, but actually it's quite impressive that this is accomplished within such a short period of time, as opposed to an order having to be canceled because a valid credit card was mistyped by the customer.
Theperplexity of this feature would be that since there are literally millions upon millions of credit cards that have been issued, that a crosscheck against all of these credit cards would imply a massive and constantly updated database containing such, which would seem to be a somewhat daunting task, as credit cards are issued by banks, by stores, and by all sorts of vendors and access to such a database that includes all credit cards would seem to be the mother-load of valuable information for those that are involved in credit card scams and fraud. While it is conceivable that such a massive coordinated database exists, in actuality, credit card issuers follow a format that has been carefully constructed so that there is the ability to apply a relatively straightforward algorithm to test the numbers that have been entered, in addition, to the fact, that the numbering convention format helps to identify the issuer of the credit card, in which as explained by stackoverflow.com, the first six digits of credit cards represent the Issuer Identification Number, so if that does not match any known vendor, the card is invalid. Then the following digits are the actual account number of the given person's account, and the final number is the checksum, in which these numbers in aggregate are tested using the Luhn algorithm, a relatively straightforward algorithm, which will be able to within the blink of an eye, determine whether the credit card number is valid by such a check.
All of the above, pretty much doesn't mean much of anything to the average consumer, because like most things accomplished on a given day, they sort of just take it for granted, but it's actually quite impressive, it does its job, because mistyped entries occur over and over again, each day, only to be instantly flagged as such, and then corrected by the customer.