I have just seen the check-box for community-wiki for an answer. For a question, I can understand, but for an answer?
Your answer might be something that you want other people to edit and augment. For example, a list of books on a particular topic, or a list of technologies that satisfy a particular niche.
In this case, community wiki is the right option since it lowers the bar for other users to add useful details.
I've occasionally made answers Community Wiki if they've been improved by many people - e.g. if people have added a variety of comments, and I've edited the answer to include them.
I'd also use CW for an answer which has just culled the best bits of a variety of other answers.
In both cases it leads to a more Wikipedia-like situation: an answer which is genuinely community driven rather than the work of one person, and which becomes the "ultimate" answer to that question.
I tend to make my answer Community Wiki if I feel the question should be CW, and isn't yet.
If you know the community isn't going to like your answer for whatever reason, then you can save yourself some grief by making it CW.