Search Twitter for #screenwriting and you're bound to run into this mantra concerning screenplays and structure:
Screenplays are structure, and that’s all they are. The quality of writing—which is crucial in almost every other form of literature—is not what makes a screenplay work. Structure isn’t anything else but telling the story, starting as late as possible, starting each scene as late as possible. You don’t want to begin with “Once upon a time,” because the audience gets antsy.
As someone deep into the Dramatica theory of story--which helps writers structure their stories and organize their thoughts--this idea warms my heart.
Yet it should be said that all stories are structure--not simply screenplays. I read an amazing book last year, All the Light We Cannot See by Anthony Doerr, and it was so well structured I could have sworn the Author knew something about Dramatica . But then, so does Shakespeare. And so does Harper Lee. Hamlet's storyform is so solid, you would think Will had a time machine. Same with To Kill A Mockingbird.
Great stories, the ones that last, communicate their message by mimicking the mind's problem solving process. The method by which that message reaches the audience--play, novel or screenplay--is inconsequential and solely up to the discretion of the creator. The function of the artist then is to embellish the structure by breathing their own unique vision into it.
As covered first in The Religion of Story Structure engineer Buckminister Fuller engineer once said “The flow of energy through a system tends to organize that system.” As story receptors our minds play an important part in the communication of narrative: they organize the story into a familiar system, or structure. So it isn't so much that the format of screenplays require structure as our minds do--for any story.
Ignore the structure of the mind's problem-solving process and you risk alienating the millions of minds looking to your story for meaning.