Not so useful on the iPhone, but awesome on the iPad.
There are times when I need to access my pc for some reason or another. I try to keep most things in the cloud, accessible from apps on my iPhone or iPad, or accessible via the web. Occasionally I’ll need a file on my pc, but more often I need to do a task that cannot be done on the iPad or iPhone, and I don’t have a laptop (or at least not one with an Internet connection). Using the iPad and a 3G connection I can use the PocketCloud app to have full control of my pc.