I find it useful when combined with other tags. Sometimes I can almost spell out the title with the tags.
An example of the top of my head..
Say we have someone asking how to resize a batch of images for their website design. I would usually tag it with:
website-design resize batch-processing images software-being-used
Leaving out the images tag here would make the information or the context created by the tags not as complete as before. Like so:
website-design resize batch-processing software-being-used
Additionally, if we burninated the images tag we would no longer have an explicit categorisation on our site stating that we accept questions about images. We'd still know, but it wouldn't be explicitly stated for the search engines.