Communication to large groups seems to be the main purpose,
(I use it in my classes)
but I also like that it can be replayed by individuals if posted on a network
or published on a disk (student can review all the main points of the
lecture). When used by individuals the audio is a very good feature,
but I can't imagine this being used very well for large groups.
Which brings up the features that are my special favorites (I really
don;t think that the purposes vary much with different people)
the animation, graphs and photos
the hyperlinks (to web, inside a presentation, to other presentations)
audio, and the ability to re-play everything