Microsoft PowerPoint: Custom Shows
Custom Shows are used to produce a subset of a presentation for a particular audience. Generally, people take a copy of the file and remove the slides they don't want. That's fine, but if a change is made to a slide they can end up having to make the same change multiple times. With Custom Shows that isn't necessary.
Another use of Custom Shows is to make your presentations more flexible. Instead of having to start at slide 1 and run through to the end, you can start with a menu and deliver the presentation in any order you like.