Hopefully, you've gotten to where your specifications clearly represents the features and functionality your client wants...but do you know why they want them?
There is a danger in focusing exclusively on the technology of a site. As a project evolves, the drive for efficiency, ease of use, or the "cool factor" can reuslt in a feature set that does not effectively fulfill the project's raison d'etre.
Behavior-driven development gives us the opportunity to sanity check the features and processes of a project, and engage clients with the development of testing scripts. With plain-language test creation, non-technical stakeholders can more fully understand the system they are asking for and provide a solid framework for making technical choices throughout the project.
Developers and QA staff who are interested in less PHP-heavy ways to write tests for your site.