This is by design:
How does a post become a Community Wiki post?
There are several ways a question or answer can enter community wiki mode, and most of >these ways will occur automatically based on the rules of the system.
Posts enter community wiki mode when one of the following happens:
The body of the post has been edited† by at least five (5) >different users.
The post has been edited† ten (10) times by the original owner.
The answer's author checks the community wiki checkbox when composing the answer. Note that this checkbox isn't available to new
users. The checkbox also is not available if the question is already a
Since October 2010, there is no community wiki checkbox when asking a question.
The answer is edited by its original author, who when doing so opts to check the community wiki.
An answer posted to a Community Wiki question will also be Community Wiki. Note that when a question is made CW after answers have already
been posted, all existing answers are automatically converted to CW
too. Older questions did not do this, so it is possible to have a CW
question with non-CW answers.
The question generates more than 30 answers (15 on Super User and Programmers, 60 on Code Golf). In this case, the question and all answers
will enter community mode, as will any future answers.
A moderator has reason to believe that the question serves better in community wiki mode - if you believe your question should
to a Community Wiki, you may flag it for moderator
† As far as what constitutes an "edit" for community wiki, only edits to the actual body of the post will count
towards automatic community wiki. Edits to the title and tags, as well
as rollbacks, do not contribute to automatic wikification.
* note that the rules outlined above are based on observations; the
actual behavior of SO is subject to change without notice, and may
well have done so since this was last updated...
What are "Community Wiki" posts?
Source What are "Community Wiki" posts?
As to why it's there - here's my take:
I see it as a way to encourage you to make a few, substantial edits to the post to improve it and then leave it alone rather than continually tweaking the post with a few characters at a time.
It's also there to discourage you making trivial edits just to bump the question back onto the homepage in order to try to get more votes.
If you think your edits are useful and you're only editing to improve the post without thought for the reputation you might earn from repeated exposure of the question then flag the post for moderator attention using the "other" option and we'll consider the request to "unwiki" the post.