"What need is there to think of these events as having three acts? None."
Why the 'three-act' structure? Why not the 'three-part' structure? The 'five-act plan' or the 'ten-sequence' tale?
It's purely academic.
First there were stories. People studied them and found similarities in those that worked, elements that seemed to lack in those stories that didn't. To be able to talk about it, they gave those elements names.
It's that simple.
Aristotle talked about 'beginning, middle, end', or rather: beginning, complications and denouement. Theater has continued using this rough three-act structure.
In the late seventies, Syd Field built further on this and he designed 'the paradigm', a 'three-act structure' specific for movies.
Since then, many have studied the structure of films and refined that crude framework into something far more practical and sophisticated. Beyond Aristotle, but firmly grounded in the foundations he built.
The motivation to study the components of story - for me and many others - has always been partially a scientific curiosity into 'how stuff works'. The three-act structure has proven to be a handy tool.
But the other motivation has always been: money. A better understanding of how audience perception works, may result in a more successful approach to screenwriting. Good business for screenwriters and producers.
Plus: with hundreds of thousands of aspiring screenwriters around the world, there is business potential in selling your ideas to this group. Syd Field soon found out after the release of his book SCREENPLAY.
Those that came after him learned that merely re-hashing old models won't work; you will need to come up with an improvement of the existing theories. That's one reason why authors keep putting their own spin on the material.
On the other hand, we have to constantly update our understanding of story structure for the screen as audience expectation changes. Cinema goers and television viewers become more and more demanding.
Still, the whole damn thing is entirely conventional.
The only purpose is for you to find a way to improve your story. And by 'improve', we mean: increase the chances of reaching a wider audience, according to principles that can be learned.
McKee says something like: these principles don't say "You MUST do this." They say "IF you do this, then...". In other words, these principles have been empirically deducted from studying stories that work.
Scientific? Oh yes.
No-one cares whether you have three acts, eight sequences, twelve or one hundred and eighty-eight journey stages, as long as it works.
Why to speak of three acts? Because if you don't, and you still want to talk story, you'll have to come up with an entirely new system. And convince the rest of the world to use it.
If, like James Bonnet, you don't want to use the three-act structure, go for your life. You may well achieve the same - or even better - results. But when it comes to discussing your work with others, you may find yourself in a foreign country. And no-one speaks your language.
You may find it's a pretty lonely world out there.