What sort of a question is that?
Essential? To spell out to whom? Yourself? Think about it... If you've written a story -did you spell it out before you were able to write it? And who cares if you (or anyone else for that matter) did or didn't or does or doesn't.
Answers like that make it easier to understand why Mr Hadrian built that wall.
Is it essential to spell out “what the story’s about”, before it can be written?

Answers like that make it easier to understand why Mr Hadrian built that wall.
I do think you need to know your protag and their problem, bare minimum. It can be really simple, ie, A girl who's getting bullied at school decides to take revenge, or, a man has 12 hours to save his family from a kidnapper, but you need some idea of the story's general form. Otherwise, I don't think you're writing a story, you're doing a character piece. Nothing wrong with that if that's how you find your story, but it's not a story unto itself.
I'll be the opposite here.
Absolutely, Yes.
Now, "spell out" varies per person, and I don't think you need to know every detail before you write it. The story will develop as you discover more about your characters and their problem.
I do think you need to know your protag and their problem, bare minimum. It can be really simple, ie, A girl who's getting bullied at school decides to take revenge, or, a man has 12 hours to save his family from a kidnapper, but you need some idea of the story's general form. Otherwise, I don't think you're writing a story, you're doing a character piece. Nothing wrong with that if that's how you find your story, but it's not a story unto itself.
What's it going to be able to do, Ray, outline itself?...
On the other hand, I know exactly what my third book is going to be able. Beginning, middle and end.
Responses like that make it easier to understand why we shipped all the crims off to the other side of the world.Answers like that make it easier to understand why Mr Hadrian built that wall.
Maybe some people can truly write an interesting novel without working out the kernel of conflict that drives the story, but I sure as hell can't.
I'll be the opposite here.
Absolutely, Yes.
Now, "spell out" varies per person, and I don't think you need to know every detail before you write it. The story will develop as you discover more about your characters and their problem.
I do think you need to know your protag and their problem, bare minimum.