BKrenning

The starch is used to put the body back in washed fabric, makes cutting easier, helps control bias edges, makes piecing easier, etc. I only starch the fabric I am using at the time & if I only plan to use part of the yardage for my current project--I only starch part of it.

I use sizing on light fabrics as I have a tendency to scorch some brands of starch but does give it some so it doesn't try to crawl away when cutting & sewing. Flannel loves to crawl away so I always starch or size it heavily.

Sizing doesn't make the fabric as stiff as starch and it costs more.

Starch comes with scents like sizing or you can add your own.

amma

Starch if the fabric pieces are going to have bias edges or to stabilize the fabric, like for OBW or S&W cutting.
Sizing to put a "little" body back into the fabric.