The main reason I’ve spatchcocked is to get chicken fat cooked potatoes. You line the pan with sliced potatoes, and elevate the chicken above on a flat roasting wire thingy. As the flattened chicken cooks, the rendered fat evenly bastes the potatoes. Potatoes also prevent smoke from forming when hot fat hits an empty pan.
I seem to remember it being easier to evenly apply seasoning under the skin with a spatchcock, but I’m not sure that makes a big difference once cooking spreads things out.
