Hi David,
The goal of the tags system is to allow users to "tag" their posts with terms which other users may use, creating indexes of similarly-themed posts which can be browsed (ie, I want to see all posts that people have tagged as being related to "plugins"), A user might be reading a post they find interesting, and want to quickly access more posts dealing with similar subject matter - they can do so by clicking the tags that post's been marked with.
As for the tags on the GT support forum in particular, a couple of us GT staffers have started tagging posts dealing with common GT product support and customization issues, just to make those posts more prominent and accessible.
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bruce@gossamer-threads.com