Users just lean back and enjoy the show; they get everything explained to them step by step, without having to click, search for descriptions or learn the navigation. The movies are usually pretty short and get directly to the point; they are mostly formal but can be entertaining, too.
But please make sure that videos are an alternative presentation of (and not the main or only!) content on your website. Not every user has broadband access to the Web, not every user is willing to watch a video (e.g. because he or she may have a radio or music playing in the background), and not every user has Flash and JavaScript installed on his or her machine.
note:: excerpts from this blog post are taken from Smashing Magazine