Is it the best practice to extract an interface for every class?

Question

I have seen code where every class has an interface that it implements.

Sometimes there is no common interface for them all.

They are just there and they are

Answer 1

The interfaces are good to have since you can mock the classes when (unit-) testing.

I create interfaces for at least all classes that touches external resources (e.g. database, filesystem, webservice) and then write a mock or use a mocking framework to simulate the behavior.