All of my paintings start with some sort of underpainting. This is coating a canvas with a color prior to the painting of an image. All further painting will be on top of that. Many artists use a soft beige color.
I use red.
The reason for this varies from artist to artist, I like to make a mark. A way to rid myself of the perfect white canvas staring back at me.
Also, I like the depth it adds to the colors I will paint next. A richness and fullness is imparted to them.
Some mornings I start with a day painted red. Everything that comes in the following hours adds depth.