Credit card payment

It's relatively easy for an interface to recognize the type of credit card being used based on the way each service (Amex, Discover, Mastercard & Visa) formats their card numbers.

A subtle interaction like this can help build trust with your user by recognizing their inputs. Thus, it helps to legitimize your product or service.

This is really important in a world with seemingly endless (and ever changing) products, apps, services, etc.