Useless interfaces

Question

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

Answer 1

It can be very good practice to set up an interface as a spec to code your class to.

If you determine the public methods/functionality that your interface will have you can lock that in place. Then it becomes much easier to code a class when you have a clear functionality in mind for it.

I feel it is more important to make writing good code easier than keep the code base clean.