Customer protection- credit cards allow you to reverse charges on defective or fraudulent purchases if you are unable to find recourse with the vendor. They usually side with the cardholder as long as a basic paper trail of a good faith attempt to resolve the issue with the vendor is included.

Also, credit cards often include insurance on purchases. For example, one of my cards will reimburse my hotel/car rental/plane tickets in certain cases of trip interruption if I purchase those items on that card.

There are similar protections on debit cards/chequing accounts here in Canada thanks to our strictly regulated banking system.

Failure condition - your conceptual exposure and, at least in the US, legal exposure is more favorable in the face of fraud.

Would you rather have someone thieve a dollar from your wallet or fraudulently assign you a debt?

Ignoring the obvious one that you can pay for things you don't currently have enough money for (bad idea in general), I pay for almost everything on a visa card for travel points.