You try a thing, learn that it doesn't work the way you envisioned and you modify it accordingly, informed by new info. If you do that well, the result is a functioning business.
Some creators achieved great success working like that, such as Fernando Meirelles, Larry David, and Woody Allen.
I'm much more of a planner myself - like Spielberg and Hitchcock. Not that I'm even close to them, of course. But I admire their method. It is not uncommon for most things to occur precisely as planned, sometimes to the second. But you gotta be prepared to think on your feet. I think that's the message behind those sayings.
“Plans are worthless when the fighting is once begun, and all depends on the inspiration of the moment.” (1877)