We should strive for a wide range of test cases. Real testing is done when the software is tested against a wide range of user inputs. Code coverage is no indicator of response to cases.
Unit tests are a fantastic way of implementing test cases. I am of the opinion that most bug PRs should start with a unit test, if nothing else, a persistent reminder that: hey BTW, your user is going to input this garbage, so any logic you implement ALSO has to be resilient against that garbage.