starch (can) make your fabric hold/stay better for more precise cuts- it helps give the fabric body/stiffness making piecing easier- and gets out those wrinkles-
some people simply spritz with a little water- or use steam- or use sizing (which is like starch but does not give the stiffness) and some use starch- it is a personal choice-
i've been using starch for 50 years-so prefer that when water is not enough-
it washes away when your project is finished and laundered.