After editing your iMovie to perfection, you'll want to show it to the world. Sure, you can preserve your work on videotape, or save it as a DVD; that's fine if you want to make a handful of copies for a few friends.
But the big time is the Internet. This 400-million-seat megaplex is where the action is, where unknown independent filmmakers get noticed, and where it doesn't cost you a penny to distribute your work to a vast worldwide audience.