Answers to the daily discussions!
I think it's main purpose is to over a template for proper story pacing.
Just like "The Heros Journey" it's a simplification of what works---it's one of those things that people also do subconsciously when telling stories. We all know that we need to introduce a character, and that we need to tell the important bits of our stories, and that we need to end a story (3 act structure.) It's used because it works and because it's engaging enough to an audience.
The three-act structure is fine for most genres, but in genres like Mystery and Crime where most storytelling uses parallel timelines, and disjointed act arrangements to keep the mystery hidden, it becomes ineffective.
Most contemporary romance novels.
Master Of The Game, Kiss The Girls, Tell Me Your Dreams (all Mystery and Crime novels.)