I starch backings heavily before layering because it prevents unwanted tucks and puckers when I machine quilt. Starching the top does the same thing for the top.
Those who pre-wash fabrics often starch to restore some of the factory stiffness to fabric (there is sizing in the fabric from the factory). Starching stabilizes fabric so it does not stretch and distort as you handle it. Starching before cutting makes accurate cutting and piecing easier.
Sizing is a chemical that mimics light starching. Starch is a natural product made from corn, and can be made in varying strengths.