use it when ever you want your fabric to have a bit of (stiffness)crispness-
it is very good to starch pre-washed fabrics=before cutting (you will get much better/precise cuts)
use it when you have stuborn wrinkles
it does not gum up your needle
if will flake sometimes if you over spray-and press while still very wet-but that's not a big deal- the flakes just brush away.