Numerous reasons. Some listed here: http://www.cloudtweaks.com/web/content/Cloud-Computing-Education.jpg
But the most pressing reason at my school is we have a diversity of devices, platforms and OS.
Rather than prescribe or provide a set device or model for students and staff we are employing a BYOD model. The origins of this system may have been financial but I am beginning to see it as a smart direction for personalisation of technology in education.
With approximately 50% of staff and students using macs and the rest using PCs I see a migration to the cloud as the only way to ensure equity of access to resources.
The areas of the school we are migrating to the cloud are:
Learning Management System > Moodle
Learning resources > Web tools and interactive learning objects
Communication and collaboration tools > Google Apps and Dropbox
It's early days and I have begun the process by exposing staff to Dropbox and Google Apps. By building their own personal networks and sharing resources with their peers I hope they will begin to utilise the www more and build PLNs.
Next step > Twitter and social media :)