Acrylic primer seals the work surface. If you're working on a woven substrate, such as linen or canvas, the acrylic primer will bond to the individual threads. If you are working on panel, the acrylic primer provides a seal against moisture. In either case, the acrylic primer helps to maintain the integrity of the surface you work on.
The acrylic primer protects the painting substrate from damage from paints and solvents. This is more important for oil painting than for acrylic or other media. Oils in oil-based paints can corrode and damage the threads of canvas or linen or eat into the wood of panels. A solid covering of acrylic primer will help ensure the painting lasts.
Provides a Bright Surface
The white of an acrylic primer sets a fine, light surface for your painting. This is important, especially if you paint lightly, not using opaque color on all areas. The light in the room where you view the painting goes through the colors on the painting, bounces off the light surface of the primer below and is reflected back through. This makes the painting more lustrous.
If you want less luminosity or prefer to work with medium tones at the start rather than light, add color to the acrylic primer to tone it. Use an acrylic artists' color. Gray is a typical choice, as it is neutral. Gray can be achieved with the addition of black to the acrylic primer, but other colors can be used as well. By toning the acrylic primer thus, you make it easier to work with darker tones accurately.