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

Bugs should be caught as soon as possible in the development cycle - having bugs move from design to code, or code to test, or (hopefully not) test to production increases the cost and time required to fix it.

Our shop enforces unit testing for that reason alone (I'm sure there are other reasons but that's enough for us).