Customers learn as well as Developers as product moves along

link -> Growing Objected Oriented Code Guided By Tests

For customers and end users, the experience is worse. The process of building a system forces them to look at their organization more closely than they have before. They’re often left to negotiate and codify processes that, until now, have been based on convention and experience.

I remember working on a system where we were writing automated Acceptance Tests that could be easily read by our PM. This process allowed us to easily show them where they were giving us requirements that contradicted requirements they gave us in earlier iterations. Trying to have the conversation in English made our heads hurt but showing them the two tests side by side got us immediate understanding and earned us the reply "Oh crap I never thought about that".