If you create a presentation and work with it using versions, you are not able
to compare this versions. You have to describe every step that you change
between the versions. That can be a very time consuming and inprecise work. I
think a tool to compare diffent versions of a presentation is a key feature, if
you work together in a group on that document. It's also very helpful, if you
work over a longer period on a presentation (with versioning).