It is useful to be able to refactor the code behind that interface, or 1 or more levels down the call stack, while testing via that public interface. Contract

It is useful to be able to refactor the code behind that interface, or 1 or more levels down the call stack, while testing via that public interface. Contract changes is one reason for such a code change, but there are many others - including bug fixes, error handling, handling input cases that were not well supported before, performance or readability improvement, changing to use a new version of an internal library with a new contract, and pure refactoring to e.g. extract common code, with no other logical change. it is useful to do such incremental changes supported by tests on a "publicly exposed interface" that does not change frequently, instead of tests coupled to classes that are not not part of a "publicly exposed interface".