You can paste any KIO URI into any application's file open dialog box and it'll retrieve it. So for example, you could open any KDE media player, paste in an audiocd:// URI, and it'll play the specified track from your CD.

And it also meant that any application could open remote files without having to manually download them first. Like, if I wanted to open some random file off the web in a text editor, I can just go into Kate, hit Ctrl-O, and paste its URL into the dialog box, and it'll fetch it into a temp folder and open it without me having to first download it manually. This was especially useful for PDFs, because Okular is generally way better than any browser's PDF reader (and I was doing this before most browsers came with PDF readers).