I principally use community wiki when I'm doing clean-up.
Example: The user wrote a question and THEN solved his question and put elements of solution in the question body instead of in an answer. Well then I edit the question, fix the issues and remove the solution from the question and put it in an answer. It's not MY answer so I put it in a CW post with the attribution to the author. That way the question is clean, and the answer can now be found easily in an answer.
In fact you can consider making a CW post when you are posting a (good) solution that doesn't come from you AND (very important) you want other people to help you improve it, or think other could come and improve it. For example someone put the solution in comments, but is no longer active and cannot post it in an answer, well then you post it in an answer as CW and mention who it comes from. Please, though, provide an answer that is well formatted and that fits the FAQ about how to answer!
If you take an incomplete answer from comments, and improve it yourself and make it a complete answer, you should take the credit for it and post it with your name.
You should NOT use CW to write answers that you know would get downvoted (and you want to avoid the -rep for it), or as a way to post really incomplete answers.