A credit card is currently a useless item, and often considered as junk.
A credit card has no use, being added purely for the purpose of clutter, creating more realism for the player when looting.
Credit cards can be found on corpses and in purses.
From newitems.txt (Project Zomboid directory/media/scripts/)
Weight = 0.1,
Type = Normal,
DisplayName = CreditCard,
Icon = CreditCard,
Custom higher detailed credit card image.