Use cases:

1. Programming puzzles. This way the answer can be posted the same time as the question, eliminating doubt that no answer is possible for hard questions and proving that the question is not a homework exercise (since the author already has the answer)

2. The socratic method. In your answer you can ask a leading question, and then once the reader has a guess they can click to reveal the answer.

EDIT: now that several new websites have been created through SE 2.0, this became much more important.

Websites such as gaming really need it.