Bill the Lizard said that
CW really isn't used anymore since anyone can propose an edit now
What are Community Wiki answers now for?
Related: What are "Community Wiki" posts?
If you know that your answer is incomplete and you want to encourage other users to add information to it, you can make it CW to invite others to edit the answer. Since everyone can propose edits, community wiki is not necessary anymore for collaborative answers, so it has lost a lot of its usefulness. It is now more of an invitation for other users to edit, but it is not really a technical necessity anymore.
It is rather used as a sign for others that you don't mind, and even encourage other users to edit the post. Many users hesitate to make big changes to posts belonging to other users, CW means you relinquish the sole ownership of the answer and encourage other users to add to the answer.
I've posted this a few other places (originally on the Music Meta) but since you want more answers I'll paste it here. Note it completely agrees with Fabian's answer and the blog post, though it predates them :P
Community Wiki is not:
Community Wiki posts stop giving reputation (due to upvotes) to the original poster once they are made CW. When you have 90% of the content from a post coming from other community members, for example, why would you want rep going to only one of them? CW neatly solves this problem.
This is why CW is bad for opinions, recommendations, and list questions. If someone provides a useful opinion on something, they should get reputation points if others find it useful! Those do not need to be collaboratively edited, and neither do lists; a new poster will just add a new list item as a separate answer.
Examples of Good Community Wiki Questions