Some writers start out writing a story knowing exactly who and what they are writing about and stay true to that vision from beginning to the end. Others write about one character doing one thing, then switch to another character doing something else, and then switch back to the first character or switch to an entirely new person with different action. Switching viewpoints reveals a multitude of sides to a story. Switching too many times leads to a tangled mess.
A writer works on the same historical novel for 5 years. She writes a completed draft from beginning to end in 3rd person omniscient. The time she takes researching and writing the first draft serves her well; she knows all the angles of her exotic and mysterious world and all the nuances of her major characters.
Unsatisfied with the distance created by the omniscient pov, she undertakes writing the story from beginning to end from the pov of a major male character in the clergy who is a true historic figure. Quickly, the writer knows he is the wrong choice to carry the story. She comes to me when she decided to write the story from the pov of view of the Grand Empress of her story -- a true historical figure.
Thanks to her broad and deep understanding of her story and the time and place in which the story takes place and a firm understanding of the craft of plotting, she plots out the entire story from the new viewpoint character's pov.
As she relates the scenes of the story, the writer struggles to surrender the story to the empress and release or push into the background some of the major elements that developed while writing the first draft and a half.
No time is ever lost when writing a story from beginning to end. Every draft, every dream, every scene makes for a better writer.