Code driven test automation is a key feature of agile software development, where it is known as test-driven development (TDD). Unit tests are written to define the functionality before the code is written. Only when all tests pass is the code considered complete.

Proponents argue that it produces software that is both more reliable and less costly than code that is tested by manual exploration. It is considered more reliable because the code coverage is better, and because it is run constantly during development rather than once at the end of a waterfall development cycle. The developer discovers defects immediately upon making a change, when it is least expensive to fix.

Finally, code refactoring is safer; transforming the code into a simpler form with less code duplication, but equivalent behavior, is much less likely to introduce new defects.