I think the main purpose of the cloud is for streaming video files to multiple i_Devices (especially the iPad and new AppleTV). Buy once - watch anywhere anytime, anyway. Once you get hooked on purchasing movies and seasons of tv shows from iTunes, storage (and sync time) does become a problem.
I'm thinking control.
Apple has everything most people use (music, photos, books, email, social networking stuff, iWork, Mobile Me, etc) on the farm. This is more than the suggested iTunes and old tv episodes. Those aren't compelling enough by themselves.
We wander around with our little iPads/phones/pods and simply access it. No need for storage or much computer power at all, mostly just a screen and a keyboard/input pad.
In time there may be enough fast bandwidth available to allow for almost all computing to be done remotely, to where everyone would be able to tap into supercomputers remotely. Imagine that! No need to buy a Pro do do your photoshop, no updating, no adding memory, nothing. Simply log into theirs and compute away.
There would be fees for all this, but a lot less than buying expensive equipment that overheats into oblivion and gets obsolete anyway. Money spent on the farm would be very much more profitable than the margins on hardware.