There is definitely an overview of primers and fillers here Beginnings:

Primers:

• A primer is a primer that is applied before painting. It improves paint adhesion and improves the durability and appearance of the top coat.
• Primers help seal surfaces, prevent stains from penetrating and ensure a smoother and more durable paint surface.
• Use primer on bare surfaces, new drywall, stained or porous surfaces, and lighting in the dark.