@RonW, Hi Ron,
It’s an old school way of working to add interest to the paint. We called it Shadow Painting back in the day. Now it’s called Post-Shading etc.
Purpose to color contrast, color variety, (what’s called modulation now days) highlights, helps pop details and lays ground work for the next step washes & dry brushing.
Several members had asked about it. We have an active thread for discussion. Please stop by and check it out.
Old School Shadow Painting Finishing Primer