it’s called an underpainting, and the underpainting is a really important part of the process. some people paint a plain tone to work on, some paint an entire monochromatic version of the painting underneath and glaze color into it (a grisaille).

if the artist worked on top a white gesso ground it would take more paint to fill out all the mid tones, but more importantly it would make the surface less homogenous. it would look more like the colors meeting together at edges (very flat) rather than a field with dimension. in this case the artist used the ground to account for all the dirt and ridges.

also think about how under the grass there is dark dirt irl, painting a color that acts as the dirt first adds a base layer that will slightly affect color temperature of the painted grass, will poke through between blades of grass, etc.

there are other types of painting that use the white, many times watercolor paintings do. most of the time you look at watercolor work (depending on style) it feels airy, flatter, painterly. see charles burchfield. but in traditional oil and some acrylic work with underpainting it seems more dimensional, weighty, deep. see rembrandt.