Ever had to move all of a file server's files to a new server? Perhaps consolidating several file servers into one, or perhaps moving the files into a virtualized file server? It's a pretty awful process, isn't it?
First, Explorer just doesn't cut it. You're not going to drag-and-drop files from one server to another, because it'll take nine hundred million years for everything to copy. Most administrators probably rely on the good old tape backup approach: Back up the files from the old server, and restore them to the new server.
Doesn't always work. While modern backup and recovery solutions are perfectly capable of restoring permissions along with files, stuff just isn't always that simple.