It helps to display the card type while you type because then I know that I am typing something reasonable and I know that I am not entering incorrectly. Of course, the entire card number could still be wrong which can only be validated by the server/backend processor but overall, displaying the type gives a level of "instant" comfort. It also gives users peace of mind that they system knows what they are doing. Think non technical users who don't understand the diff b/w client side vs server side validation. For them, it will be comfortable to see that the system interprets their card type which might give them more comfort. No data to back this up but just something I might think as a user.

I get your point, but to tell that a card is a VISA you just need to enter "4". I don't think it makes much of a difference really, just avoid showing in card type info and no one wonders.

It might be better to let the customer pick their card type, that you could also better inform them that you do not accept their card type.

One scenario where immediate detection is helpful is for unsupported card types. The server detection would require users first fill all their card data only to discover they need to start over.

I'd say in part because you want to give people a UX they are comfortable with. Sites for years have been asking for "Card type", and so a user may suspect something is wrong if they don't need to enter it and show no indication you know what it is.

That seems unlikely, but I've certainly been wrong about UI questions before. I'd be very curious if anyone has any data supporting this idea.

No data, but as a user I do expect to see a card type selector or indicator, even though I rationally know its superfluous.

Unfortunately I don't have actual numbers for you, but a company I used to work for that did a fair bit of CPO had good data that removing that selector reduced conversions (the most interesting thing to come out of that study was that, at the time and with our client base, removing the actual card logos (e.g. leaving only the textual names) increased conversions).

I've seen sites (can't remember which ones off the top of my head) that do autodetect your credit card type while typing it in. I wasn't phased by it.

Importance on the front-end isn't that high. Though we use it to change messaging around where the CVV is located. You do need to detect it on the backend as you have to pass it to the payment gateway, i assume the reason is to determine which network to put the transaction on. Some gateways take care of this step for you but most of older or lower level ones don't.

Maybe so that the user has something visual to show them they haven't miss typed that start of the number?