Why are functional tests not enough? What do unit tests offer?

Question

I just had a conversation with my lead developer who disagreed that unit tests are all that necessary or important. In his view, functional tests with a high enough code cov

Answer 1

It can be a lot more difficult to find the source of problems if a functional test fails, because you're effectively testing the entire codebase every time. By contrast, unit tests compartmentalize the potential problem areas. If all the other unit tests succeed but this one, you have an assurance that the problem is in the code you're testing and not elsewhere.