The new version of Keynote allows for expanded use of the canvas workspace, the area outside of the visible slide. You can use this area to store items that you don't want people to see, but may want to add back to the slide later. You can also use the canvas to better orient and set up your animations that start or end outside of the slide.
Another use:
I usually give my slides in PDF format to my class and it's useful to supplement slide information using the Presenter Notes, which can be added in the print options.
The problem is that sometimes the 'Presenter Notes', really are just for me, not for my students. This canvas workspace feature allows us to print slides to PDF, include the 'Presenter Notes' (for students) and the notes on the canvas workspace get left behind (so, just for me).