Move to iTunes moves the audio and video files dropped onto it to your iTunes library. When the original files are on your hard drive, this happens nearly instantly, even for massive video files, and leaves no duplicate files on the drive. In the case of video files, it also sets them to resume where you left off, and so they will not play during shuffle play.
Why? The primary use of this for me is to add DVD movies I've converted for my iPod with Handbrake to iTunes, without having to wait a long time for them to copy, without having to manually trash the duplicate original, and without having to manually set the "Remember playback position" and "Skip when shuffling" options every time.