Why do DVDs have trailers?

I was sick for the past two days and I was watching some old movies and I realized that all of the movies have trailers. That is so frustrating! I realized that needs to change.

I was watching an old movie and I saw all of these old movies that I have never herd of. I think that is so dumb of the movie companies because if they allow trailers on their movie they are known as, "that movie with all of the wierd trailers". I totally disagree with the whole idea of trailers becasue they are, at least in my opinion, totally ridiculous.

One good example of a movie that does NOT have trailers is Avatar. I remember an interview with James Cameron that I watched and he said that if he had allowed trailers then it would take away from the entire movie. That is exactally how I feel about trailers... They are a big take away from the movie itself.