Unit tests aren't for asking the right question. There code is there and readable (hopefully) so that shouldn't be an issue. Unit tests are there to ensure when you need to make a change you don't include a regression (moving code backwards). Missed something that the function was previously doing.

They aren't useless and have a clear place in testing. Unit tests alone can't guarantee a fully tested system, but they minimize engineer error when working on various pieces of a code base, but this is predicated on cleanly separated and testable code pieces.