The importance of preliminary sketching

https://www.facebook.com/ArtistOfTheYear/videos/362104205109908
David found this short clip from Portrait Artist of the Week/Year.
DAVID: Most of the Great Masters in Art and Music make lots of prelim work before shaping and editing. I doubt whether great art ever arrives fully formed.
PETER: I guess some people are able to knock out perfect lines of prose off the top of their head, but most people need to do lots of drafts, editing, rearranging, figuring out. Same with art I think.
JOHN: All very neatly put by Tai. A very useful summary of reasons for preliminary sketching:
- Relaxing nervous energy
- Getting hand and head in tune
- Working out composition
- Seeing the subject from different angles
- Personalising
Number 5 is a very interesting point. In the sketching process you begin to adjust your marks and nudge your interpretation into your own inimitable style. A direct drawing with no prelims is much more of an unstylised response.
Which is why a lot of the drawings displayed at the end of a life drawing session look rather similar, regardless of who did them.