Secondaries with the Pros - The Art and Technique of Digital Color Correction - page 207

Fig. 6.2 Source image without vignette, image courtesy of Artbeats.
Fig. 6.3 Vignetted image with very strong vignette effect, image courtesy of Artbeats.
For the colorists that used this technique in the sessions I watched,
the vignetting clearly improved the final image, which I could confirm by
disabling the secondaries with the vignettes in them.
It makes the image seem to have more depth to it. If there are big
bland areas, like a gray sky, it gives more texture to it, and it focuses the
attention where it's supposed to be. Festa agreed with this assessment: