Useless interfaces

Question

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

Answer 1

One reason could be decoupling: if your classes are used by another developer, you can give him the source code for the interfaces, and keep the implementation detail for yourself.

If the implementation changes, and the "contract" defined in the interface doesn't, you'll be happy: your interface still describes what the class does, and nobody has to know how it does it.