To reveal in the test strip above and beyond too light and too dark... So you might know all the possibilities.
Secondarily, because it's an f/stop-like sequence.
It's NOT loaded with hogwash, though you might easily know everything it offers to teach. Unique in the book is his coverage of retouching with New Coccine, flashing to darken paper edges dramatically and silhouette your subject, combination printing, borders and cut-and-paste montage.