They can also be used as a source of payday/personal loans, but that strikes me as suboptimal. Justifying the existence of credit cards as a source of credit doesn't make sense. One, why must the credit be tied to a card? Two, how often is a responsible spender gonna need that card as a lender of last resort? In fact, the idea of a "charge card" predates the idea of attaching a revolving line of credit to one, and you can still get charge cards from American Express, among other firms.
The only argument in favor of credit cards is the freedom-oriented one that people should be free to have easy access to run up revolving debts. I'm fine with this argument, but let's not obscure the issue by pretending that a significant number of people actually need access to that credit and that they need it in card form. The utility of credit cards lies in making payments, not having credit, and they're clearly an inferior solution--the space is ripe for disruption.