I believe that if a question is correctly tagged, it may be very useful to all users of the site, since it makes finding questions much easier. Another advantage is that users can easily follow questions from their area of interest.
Tag-wikis are helpful in tagging question correctly, in particular, for some tags that seem ambiguous, tag-excerpt and tag-wiki usually explain the intended usage.
Often a tag is created without a tag-wiki and when someone later tries to create a tag-wiki he is not sure about some details or the intended usage. I've open this thread precisely for this kind of questions. (Of course, some tags with unclear meanings may deserve wider discussion and it is better to open a new question for such tags.)
EDIT: Perhaps to explain better what I intended as a purpose of this post (and also to address this comment). Editing tag-wikis is somewhat different from editing posts. When a post is edited (to improve TeX, formatting, to retag or for any other reasons), it is bumped and the edit is visible to many users. Similarly, if a new tag is created, it appears here. Even if some user edits a post with best intention, it is good that other users see his edit and can check it. (And if they disagree with the edit, they can do whatever is needed.)
But when a tag-wiki is edited, there is nothing similar which would attract attention to it. So this thread was intended for something along the lines: I've edited the following tag-wiki... I did my best, but since I am not entirely sure, I'd be glad if someone checks my work.
So basically when something new appears in this thread, it serves as a notification that some tag-wiki was edited, and perhaps some of the users will check the edit. (I don't say that all edits to tag-wikis should be collected here, only the ones where the editor thinks that it would be good if his work was checked by other users.)