Why Is It Important?
Images that don't contain copyright information may be deleted, since we have no way of knowing whether it's legal to use the image or not. A tagged image will automatically have standard copyright text included, and it will be given a category regarding its copyright status. Further, automated tasks, such as moving images to the Commons, are much easier with tagged images.
But there's a more immediate reason to tag images. Mandriva Linux plans to include a static copy of Wikipedia in their next Linux distribution. (See meta:Wikimedia and Mandriva for details.) Images that have not been tagged will not be included. It's great that Wikipedia is being promoted in this way, and we want it to look as good as possible.
Read more about this topic: Wikipedia:Untagged Images