During my time working on websites for a retail company the imagery of the credit cards accepted were considered important. They would even be on pages that just mentioned taking payments before you get to the actual input page.
One reasoning is that it is a sort of reassurance, much like the stickers you see on doors of retail locations that show which cards they accept. It's a reassurance in the idea that if you're deciding if you want to make the purchase or not, that the site will have no problems in accepting the payment option you would like to use. Plus, in a strange sort of way, it implies the site is a valid on-the-level company because surely a credit card company would come down hard on a scam site for using their copyrighted visual identity.
There's not much valid reasons other than it's a visual thing for customers. Although I always suggested using the method of displaying the type of card after starting the number, that goes against the reassurance thing. If a customer has two different branded credit cards, they know up front if one or both will be accepted. Otherwise they have to start typing to find out, which is work for the customer. You always want it to be easy for the customer to spend money, no second guessing.