Protection and convienience
I am now on my second card case. I dropped the other one more than I would like to admit.
I like being able to carry a couple business cards in the back, and the outer slot is where I put the steel plate that my Scosche mount uses in my truck. I can easily tell if I need to add a couple cards so that I always carry some just in case I get asked. I have carried a credit card or two as well, but I find that business cards are critical for me.
I might be more inclined to carry my credit cards if the case was RFID blocking.