The benefits of testability as a focus of requirements gathering and automated tests in the source tree as a concrete artifact of that requirements gathering is:
(1) If something is specified well enough to be automatically tested, there is clear agreement (and not superficial agreement hiding different interpretations).
(2) If automated tests are created and in the source tree, the unambiguous knowledge in #1 is preserved which makes resolving future questions of expectations and intent of the existing code base easily resolvable.
Comments
The benefits of testability as a focus of requirements gathering and automated tests in the source tree as a concrete artifact of that requirements gathering is:
(1) If something is specified well enough to be automatically tested, there is clear agreement (and not superficial agreement hiding different interpretations).
(2) If automated tests are created and in the source tree, the unambiguous knowledge in #1 is preserved which makes resolving future questions of expectations and intent of the existing code base easily resolvable.