Trailers, teasers, ads, posters, public demos, they are all about getting people to know about an upcoming work, and to get people interested in seeing/reading/playing/listening to it. Of course hype is only half of this. Upon release, people have to like the work, but people still have to know it's out there. It can be a tough task, one that marketing departments and advertising firms have spent years trying to perfect. But a well-done campaign can be the difference between blockbusters and flops.