The main value of three act structure is that it allows writers to see how incidents radically change their story depending on where they are placed.

"The owl of Minerva spreads its wings only with the falling of the dusk." _Hegel

Let me preface this with saying that I'm a very inexperienced screenwriter.

I feel that all stories have a retroactive continuity and that our brains are hardwired to find a narrative . Our brains being great pattern finders will imply a structure even when there is none.

My background is in experimental film-making and i once watched a piece by Stan Brackhage frame by frame on a couple of hand cranked rewinds. There was 20 ft of of nothing but black film, then a single frame of a moon just over top some swaying pine trees. At the end of the roll just another 10 ft of total darkness.

After it was finished the narrative thought experiment awoke in me. Picture this: Your walking through a pitch black dense forest at night . Suddenly there is a break in the foliage and the moon appears. The light draws you in like deer in headlights, but you keep moving into the wood, into the darkness .

Stan was a non-narrative film maker , however that didn't stop me from conjuring one out of the ether. It is just the nature of the human experience to do so.

In tabletop we have a concept that is known as a Railroad. The railroad is structure of play that guides the PC's through the experience of the design of the game crafted by the GM. The big problem is that it hardly allows for player choice to manifest having any bearing on the world. The three act structure might implicitly have that effect on the characters in a screenplay.

For me the act of writing a screenplay is like a game of chess where you play both sides yourself but in the mode of Game Master and Player Character. Taking on both roles as the needs demand. As a tabletop game this

" Some people legitimately might not realize that a story could take any direction, not just the one that was chosen." 

really rings true. Especially if we allow the characters choice to really matter in how we shape the adventure, world build, and choose encounters that resonate as possible . That's how I understand

" There are some writers out there who obsess over page counts and obligatory plot milestones to the point where the structure chokes the life out of the material. Don't be that writer. There are some writers who instinctively just get it, and don't need hacky terms to get a sense of material."
/r/Screenwriting Thread