The Copy Cat perk allows you to configure a checkbox which, when clicked, will copy the values from one field to another. This is particularly useful for situations like specifying a billing and shipping address. There is a good chance the user would desire to enter the same address for both fields. With Copy Cat, you can configure a checkbox to copy the user-entered values from the billing address field to the shipping address field.
This also supports simply copying the value from one field to another after the user has entered a value. For example, the user could enter their first and last name into a field and you could automatically populate this into the Name input on the Credit Card field.