Security ... think about it!
Take driver's licenses, for example. Originally designed to demonstrate a credential -- the ability to drive a car -- they looked like other credentials: medical licenses or elevator certificates of inspection. They were wallet-sized, of course, but they didn't have much security associated with them. Then, slowly, driver's licenses took on a second application: they became age-verification tokens in bars and liquor stores. Of course the security wasn't up to the task -- teenagers can be extraordinarily resourceful if they set their minds to it -- and over the decades driver's licenses got photographs, tamper-resistant features (once, it was easy to modify the birth year), and technologies that made counterfeiting harder. There was little value in counterfeiting a driver's license, but a lot of value in counterfeiting an age-verification token.