Useless interfaces

Question

Why would you ever use an interface if you are only going to have one implementation of it?

Answer 1

When I work with someone else on a project, for instance if I do the front end (Web application for instance) and the other person does all the database work, we start by writing an API. The side that faces me is all about the problem domain: classes for User, Administrator, Employee, SKU or whatever. Then we can work independently; she implements all the interfaces and I write the code that uses them.