Starching stabilizes fabric so that you can get more accurate cuts and also piece more accurately (less stretching of the fabric edge). Stiffly starching backing fabric before layering the quilt sandwich prevents tucks and puckers underneath when you machine quilt.
I don't use spray starch because I always seem to scorch it. :oops: I lay fabric on my kitchen island, use a large wal painting brush to "paint" on a 1:1 solution of Sta-Flo liquid laundry starch and water, toss the saturated fabric in the dryer, then iron with steam.
I usually don't starch fabrics for the top because I don't prewash. Fabric comes from the store with some sizing and starch already in it, and I find this is enough to keep me accurate with cutting and piecing. I do prewash and dry flannel fabrics, though, because they shrink so much. Since washing removes all the manufacturer's sizing, I use my starching method above for washed flannel. It makes my flannel piecing very accurate.