Theoretically, nobody opens their mouth without having something to say and yet, many screenplays have a beginning, middle, and end but no theme, point, or purpose.
I'm not talking about WHY a writer writes. I'm talking about WHAT a writer has to say. Nor am I even even hinting that a screenplay has to change the way people see the world but a story without an underlying thought or prevailing idea is like a soda with no bubbles -- or bubbles without the soda.
If I were a respected screenwriting guru who'd never sold a screenplay or been produced but published a book on how to write screenplays and get produced, I'd call the book "Make it Say Something".