The honest answer
The accepted answer's first sentence is that this became the convention a decade or two before the question was asked, at a point when nobody knew better, and conventions persist. The number's opening digits do identify the issuer, so as a data-collection step the field is redundant. It went on to connect the practice loosely to the compliance arrangements merchants operate under, but the primary explanation offered was inertia.
The reasons that still hold
- It advertises what is accepted. One answer made the case from the customer's side: seeing only two brands listed saves you typing a card that was never going to work, which matters because some merchants decline particular networks over processing fees.
- It makes validation explicit. Different networks use different numbers of digits, and one answer preferred stating up front what the system should expect rather than inferring it.
- It improves the error. If someone says one thing and types a number of the wrong length, the mismatch can be caught and explained specifically.
The reason people outside a few markets raise
One answer, noting it would not be popular, pointed out that a very large share of online payments in China route through a domestic network rather than the two brands usually listed, and that customers may not realise a particular card type is required until the payment fails. The general point survives the specifics: a form listing only the networks familiar in one market is making an assumption about who is buying.
Where this leaves the field
- As an input, it is unnecessary — detect the network from the number as it is typed.
- As information, it is useful — show the accepted networks near the field, before anyone starts entering digits.
- Those two conclusions are compatible, which is why most current payment forms display brand marks while asking only for the number.