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If you only use OmniFocus on the phone, then having a MobileMe (or other WebDAV) account and configuring OmniFocus to sync to it means that if something happens to your phone (gets lost, breaks, etc) or to the data on your phone (a bug in Apple's update mechanism has cost some people their app data when they upgraded to newer versions of iPhone apps, for instance), then you can retrieve your data by syncing it down from the server.

If you also use OmniFocus on a Mac, then having a MobileMe (or other WebDAV) account and configuring OmniFocus to sync to it allows you to share your data between OmniFocus on your phone and OmniFocus on your Mac.