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

unit tests are for devs to see where the code failed

functional tests are for the business to see if the code does what they asked for

unit tests are checking that you've manufactured your bricks correctly

functional tests are checking that the house meets the customer's needs.

They're different things, but the latter will be much easier, if the former has been carried out.