When Brian Eno was working with U2 and Luciano Pavarotti on the song Miss Sarajevo, he observed that opera singers are cheats. Pavarotti came to the studio, looked over the libretto and noted which lines were easiest to sing. He sang those first.
Instead of singing the operatic verse through, he did it in pieces.
Why wouldn’t he? The idea that he would sing the full verse in order and deliver one perfect performance is what the audience sees. It’s not how the work is created.
Those who start out writing a book, or any creative work, think that this is the way it is meant to be done. The professional knows that it is done in pieces, and sets to work by first understanding and defining exactly what are those pieces.
Throwing away these assumption of “the way it is meant to be done” can help in two ways.
First, it can make things more efficient by making the process easier. Breaking the task into pieces you know how to tackle can save time. It can help maintain momentum. You can structure the work to suit your flow by placing more difficult parts where you know you will be most suited to tackle them.
Secondly, it can spark creative decisions and choices in a different manner than starting at the beginning then running head first into a road block.
The truth of the matter is that no matter how the work is done, the professional or the artist understands that the right way is the way that works for them.