Just about all checkout forms display what kind of credit card has been entered (visa, etc.). There doesn't seem like there's any reason for this, other than it's what the form has evolved into.
I don't think there is really is "general" reason. From a usability perspective if the user has a Visa and fat-fingers a 5 as the first digit it can be nice if they see that there may be an error before hitting submit (although in all honest I don't really pay attention to the icons that highlight on some forms and still fat finger numbers occasionally).
In a related topic on of my biggest pet peeves with sites is the ones that force you to choose the credit card type instead of detecting. Very frustrating to hit submit only to be informed you forgot to select the type of card you are using. To me there is no logical reason behind that.
Wouldn't some other feedback actually be better? If your JavaScript can calculate the Luhn checksum, and then give a checkmark or something, you've actually done better than just checking the card number's first few digits.
Very true. But is there a reason that's better than a checkmark? It just doesn't seem like a user cares what card type they've entered. They already know since they got the card out.
