TDD Tests as System Documentation

The tests provide a working description of what the code should do, readable by any programmer familiar with the programming language

Explanation

TDD tests serve as informal but executable documentation. They provide a description of what the code should do that can be read by any programmer familiar with the programming language. Unlike traditional documentation, TDD tests are always up-to-date because they must pass for the system to work - they cannot go out of sync with the implementation.