Why? Two reasons:
1) Say you LOVE a movie, and have seen it a dozen or more times. On a long road trip, play the movie's audio, and your mind fills in the visuals. Like watching a movie while driving, without the danger!
2) Ringtones! Turn your favorite movie theme or quote into a ring for your phone!
Even better, there are some GREAT tools for this. For ripping the audio from my DVD's, I have been using DVD Audio Extractor. 30 day full functioning trial.
To create ringtones, I used DVD Audio Extractor to rip a single track, then used WavePad to rip just the piece I wanted. That's the steps I used to create this perfect ringtone from Die Hard...and my personal dream. ;-)