Words convey meaning, story and importance. In a way, the entirety of a book is dialogue between an author and the readers. More specifically, though, dialogue exists as the conversations between characters and can function both to progress the plot, as well as to color and deepen the characters.
The importance of dialogue is illustrated well by two movies about the introduction of sound into cinema: Singin' in the Rain and The Artist.
Singin' in the Rain demonstrates the importance of voice. Only with Kathy Selden's voice does The Dancing Cavalier work. With Lina Lemont, the picture is The Duelling Cavalier. The titles are telling. With Kathy, the dialogue flows and dances, mesmerizing the audience. With Lina, the words are struggling to break through and a duel does ensue, with the film's success as the casualty. Voice then is of visible importance - the right characters must speak with the right voice, including mannerisms, in order for the story to work.
The Artist also demonstrates the importance of voice. Although it is a largely silent movie, title cards throughout the film perfectly convey what material needs to be presented for the success of the audience's enjoyment. It is what is presented, as well as what is not, that creates the voice of the movie. The same is true for novels; knowing what to include, what to leave out and when to present the information can cause the story to sing.
What other movies teach about dialogue?
My best to you all,