Convenience. All you have to have is fire, after that it boils down to convenience. You could also ask, Why use charcoal baskets? You could always just stack up the charcoal. For that matter, Why use charcoal? You could always burn wood to make coals.
Bottom line is the Vortex makes cooking chicken pieces on the kettle a breeze. You can get the same results without it, but the Vortex makes it effortless. I have done a comparison of the baskets vs Vortex, and the big difference is the Vortex is taller. It comes up to the bottom of the cooking grate, so the heat is funneled up, and then comes back down from the dome. This makes for very even cooking, and you don't even have to flip them. The baskets are shorter, so more heat travels out under the food, giving you an opportunity for hot spots. This means you have to watch the cook, and move and flip to get an even cook. With the Vortex, you put the wings on, come back in 45 minutes to get them.