Anyone knows what's the plate behind the wheel of the Pilatus Porter for?
In this case though, the engine is in front, so it is likely that it is only there to reduce the amount of dirt raised when landing/taking-off so to reduce the chances of damaging the aircraft.
Federico hit the nail on the head. It’s a gravel kit and works similar to the mud flaps on trucks. On rough airstrips, it prevents rocks, gravel and other FOD from striking the airframe and damaging it during the ground roll.