What does the "Community Wiki" option do?

When composing a question on this site, you are presented with a "community wiki" checkbox.

I clicked it to see what it did, and now it looks like no one can gain reputation by answering my question and I can't change it back.

What is the purpose of wiki mode, when should it be used, and why isn't it directly covered in this site's FAQ?

It allows anyone with 100 or more reputation to edit your question/answer, instead of the usual 2000. It should be used for things like this question or this one, where it is useful for people to be able to add info without creating a new answer.