I'd imagine one of the main uses (perhaps even the intended use) would be to transparently allow addition of any kind of meta-data to a file. The reason the file size does not change is in this scenario you do not want the file to look or behave any differently lest the originating application relies on some aspect of the way the file looks.
I could imagine interesting uses in IDEs for example, where sometimes multiple files are involved to form a single unit (code file / form file, etc), which could be attached to the original file in this way so that they cannot accidentally get separated.
I also believe there is a command to find all such 'attachments' in a given directory tree, so they are not actually completely hidden. It also would surprise me if the better virus scanners are not aware of this and check these 'hidden' areas, but you could check that by purposely attaching an infected executable to a text file and seeing if it gets picked up.
