FWIW, this does not match my experience. I have caught lots of bugs with unit tests, especially in code that is fundamentally complex (because it does complex things, not because it needs polishing). OTOH, refactorings often span units because real simplification comes from changing the ways units interact, or even which units exist, so the tests have to be changed anyway.

Granted, even tests that have to be changed have some value in securing a refactoring.

If you are writing tests to check new code in tandem with writing that code (either via TDD, or some other code-test loop), or are writing tests for existing code, you can (and usually will) find and fix bugs. Likewise if you are investigating a problem and write one or more test cases to check the behaviour.

Once those tests have been written, then they act as regression tests like the parent comment notes.

On "unit tests", I view the behaviour of the class/function and all its dependencies as a single unit for the purpose of testing. I've never liked the idea of needing to mock out a class in order to test another class, just because the class being tested makes use of it. The only case where mocks/stubs/etc. should be used is when interacting with an external component like a database or HTTP API. -- You don't see people that do this mocking out a list, string or other library classes, so why should project classes be any different.

To clarify, when I wrote "the ways units interact", I was referring to units that represent external dependencies in some kind (often a database, as you said). Many refactorings change those interactions in some way.

I agree that there is no reason to mock data containers or other classes with fully self-contained behaviour.