If you've worked on the same large-ish showcase for a long time, chances are there will be some unused slides somewhere in the mix. These won't cause any real problems with your showcase, but they can clutter up your "Existing Slides" dialog, and your Slides panel on the left-hand side, making it confusing to find which slides you want to link.
Unused slides are denoted by the little "eye with a line through it" symbol, so when you're choosing slides to link to, you can tell if those slides are currently not linked anywhere in the showcase.
Sometimes, you want slides to be unused for a time, for example if you want to be relocating a submenu from one spot to another within your showcase.
Once you know for sure that you aren't going to use these unused slides anymore however, you may want to "clean them up" and get rid of them for good.
You can do this from the "Find" dialog top left of the showcase options while editing.
Down the bottom of the "Find Slides" dialog is the option to "Delete all unused slides".
Clicking this will give you a warning that you are about to permanently remove the slides from the showcase, so you have a chance to confirm before they are gone for good. Sayonara slides!