It doesn't take very long to do a preliminary tonal study, but the time spent pays big dividends. Here's a small pencil sketch that I did in preparation for a painting in Dinotopia: Journey to Chandara.
For example, the tonal study helped me plan the dark area behind the light feathered dinosaur in the lower right, and it helped me work out the chiaroscuro of the bearded farmer.
Once I get into the details of the painting, I'm making decisions at a more micro level. Without that tonal study, it's hard to see the big picture.
The original pencil tonal study appears in The Art of James Gurney exhibit at the University of the Arts in Philadelphia through November 16.
More about various kinds of preliminary drawings in my book Imaginative Realism: How to Paint What Doesn't Exist
Dinotopia: Journey to Chandara on Amazon
U.S. customers can get signed copies of both books from my website store.