When you read the entire section of the book, Martin claims that those tests main purpose is to serve as a normative documentation. The developer can read the test case and then figure out what was meant to be programmed.
While it might be illuminating in some cases, it might be not helpful or either misleading in other cases.
It's like arguing that the main purpose of magazines is to lit a fire with them, because some of them are very flammable.
Tests are good to ensure that some property of the system that held true before still holds true after some modifications to the system has been made.
