When to write a Use Case

[Colin Williams]

In episode 7, Uncle Bob talks about Use Cases and how stories become the titles for them. What I'm struggling to understand, though, is when this should occur. Doing it ahead of the iteration seems like it would reduce the Negotiable nature of the story, but when I've tried to wait until the iteration is underway people seem to really struggle with estimation.

One of the points Mike Cohn made in User Stories Applied was that the story is a placeholder for a conversation. This makes sense and helps resolve the estimation challenge to some extent, but then I begin to ask myself why we should write the Use Case at all. In other words, if writing is deferred until the team already understands what needs to be done, what purpose is the use case actually serving?