iCloud provides online cloud computing, not just storage, but a desktop and applications – so in theory you don't need anything on your computer except a web browser (and a fast internet connection).
The desktop looks good and has pre-installed the common applications most users will need. It's kind of odd setting up an email client on a cloud desktop, especially I'm connecting to GMail anyway, so I could just open another browser window to use it. However for locked down computers where iCloud is the only thing running this makes more sense.
Your given 3Gb of storage to begin with, and it certainly looks nice that Google Docs – but it runs slower and I'm not sure if it's something I'd seriously consider using yet.