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.
This is perhaps more important at main site than at meta, but tag-wikis at meta are useful, too. For example, often the tag-info on meta contains links to basic information about the topic of that tag and so it is might be a useful place where to direct users asking about that topic.
In the last few days I've created or edited a few tag-wikis here at meta. Usually I have copied the tag from corresponding tag at meta.SO, perhaps with addition some information specific to math.SE.
If you think that this is a useful thing, I invite you to joint the effort to add more tag-wikis here and at the main site.
Sometimes I wasn't sure what should be written in the tag-wiki. This is the reason why I am starting this thread - so that questions about tag-wikis can be posted here.
TL;DR If you are not sure whether the text chosen for a tag-wiki or a tag-excerpt for some tag at meta is a correct one, this is the right place to ask.