Tags are used to assist in searching.
So... for this thread, you might add the following tags... "tag, posting, thread" or something like that... single words that contain the key aspects/attributes of the post.
When searching, go to "advanced search" and you can see the tag cloud. You can search for those specific tags, rather than trying to free text and sift through a bunch of irrelevant results. If I'm not mistaken, the tag cloud makes the font larger for a certain tag depending on the number of times it is used or searched for.
Personally, I find tags useless, especially when users are allowed to make up their own tags rather than select from a pre-set list, because over time there ends up being so many tags that it makes tag searches just as difficult as free text.
However, they are nice to use to search for multiple attributes... if you wanted to find all posts related to lighting -and- FOWLR -and- 180g, you can do that much more easily with tags.... of course this is assuming that every post that has those attributes has been tagged accordingly...