Body kits aren’t a particularly elegant solution but they are also functional.
Most of the drift chassis width are over 190mm, but quite a lot of traditional touring shells only have 190mm version. If you want to put a touring shell on a drift chassis, sometime you'll need the over fender to widen the shell. The front spoiler and the rear wing provide down force in high speed. Side skirt lower the ground clearance on the side and reduce air goes underneath the chassis.
But most of the time, you need the body kit only to make the shell looks tremendously cool!