For example: if you have a website about minerals and crystals, then members can tag a specific stone or mineral in a picture. See my example here: http://xenmediagallery.com/media/albums/alfa1s-minerals.15/
This picture shows 4 different types of fluorite.
If you could tag these objects with the appropriate names then its clear to the viewer which object is what.
Its also useful to be able to click the tag and see threads and images tagged with the same tag.
For example: if the fluorite image has the tag 'amethyst' then we would be able to see which mineral is the amethyst. We would also be able to click the tag and see threads with information about amethyst and other media tagged with 'amethyst'.