But wait, can’t you make poured fondant with powdered sugar? Why go to all this trouble?

You can make an ersatz poured fondant with powdered sugar, yes. Some versions don’t even have to be “cooked”. However they’ll never be as good as a sugar syrup fondant because all powdered sugar contains cornstarch (about 3% by weight). The effect, in the case of an uncooked fondant, is to give the icing a vaguely starchy taste. Cooking gets rid of the taste, though it doesn’t get rid of the starch, which still makes the icing a bit chalky-feeling in the mouth.