Wheat flour contains starch and protein (gliaden and glutein combine to form stretchy, elastic gluten when water is added) but cornstarch is starch only, thus, gluten free. When the batter hits hot oil, the moisture in the starch(es) gets released and the batter starts getting crispy. Flour batters tend to brown more than corn starch batters (typical of Korean batters) and sometimes paprika is added to corn starch batters to give a darker color. When proteins hit the hot oil, the protein tightens up, especially if one has worked the batter a lot and created a lot of gluten strands in the batter, and this results in a more dense, more chewy crust. So a flour only batter (though this is typical of many deep South batters and the milk/flour batters of the German/Czech church picnics) has a good crunch right out of the fryer, but one that softens as the crust cools. There's still a lot of moisture in that batter, too, so it gets softer as time passes. A flour and cornstarch batter is pretty good. You'll get a crispier coating for sure. My own preference, based on working on the Gus's Fried Chicken recipe for years, and my love of Korean Fried Chicken recipes, is for corn starch (with maybe just a little bit of wheat flour). Double fried. A cornstarch crust stays crispy longer, too.