To prevent ash from forming on the soap. I've never experienced ash but its basically just a cosmetic dusty look some soap gets when exposed to air. I believe.
I let my soap set up a bit (maybe 5 minutes) and then liberally spray it with 91% Isopropyl. I don't find it always works to prevent the ash, but it does make it easier to rub off when it does form. I find if I spray it on "wet" soap batter, it leaves a weird crust on top.
