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.
From the official PC-6 Porter brochure via pc-6.com (PDF), they call this option a mudguard:
And according to a service bulletin (pilatus-aircraft.com; PDF) there were prone to fatigue:
Operators have reported failures of the mudguard supports and stone guard mounting yokes. The failures are caused by fatigue damage.
Actually - Federico didn’t exactly hit the nail “squarely on it’s head” The main function of these devices is to spread the load, ie reduce the weight per square unit by increasing the ground contact patch of the tyres. Many bush airstrips has a very loose / soggy sub-soil, and I’ve seen first-hand that a heavy bush-plane can sink through to it’s belly while sitting still or moving slowly when operating in unimproved jungle-airstrips in Asia or Africa. Think “snow-shoes” in the Arctic or wheel-penetration skies, like the ones I flew in Antarctica. If the “ground contact patch” of the tyre itself, starts to break the surface, it will sink down until the device touches the surface, whereupon the load is spread out to much larger area, and the aircraft doesn’t break through.