These are photos of Tecnam P2006T rudder trailing edge. Why it has such flat plate mounted at the end of the rudder?
What seems be the case here is that a truncated trailing edge has a flat plate riveted on (called a double Gurney flap). This places the plate perpendicular to airflow (more or less). This, in turn, has the effect of increasing air pressure on whichever surface of the rudder is deflected into the airstream, giving a similar effect to increasing rudder area - but without the added weight of extending the trailing edge of the rudder.
Since weight in the tail is generally bad - because of the moment arm from the aircraft center of mass, a kilo added to the tail requires five kilos or more in the nose to return to correct balance -- this is probably done mainly as a weight saving on an aircraft that (as a twin) needs good rudder authority. As a bonus, it reduces the amount of deflection required for a given yaw force, which makes trim inputs more effective.
It's a double Gurney flap, but I'm not sure how it helps, here.
The Eurocopter AS355 TwinStar helicopter uses a double Gurney flap that projects from both surfaces of the vertical stabilizer. This is used to correct a problem with lift reversal in thick airfoil sections at low angles of attack. The double gurney flap reduces the control input required to make the transition from hover to forward flight.
In the context of aircraft, it's misleading to call this nonmoving surface a flap, of course. But the term and the concept came from "hey, Billy Joe, let's rivet this on and see how she runs" motorsports, a culture that wasn't fussy about nomenclature.
Here is another consideration.
At the aft end of an aerodynamic body, we taper the body so the streamlines in the air flow over it do not separate, because of the flow separation drag penalty.
But in doing so, we increase the length of the body's tail end, which increases wetted area drag.
There then comes a point where truncating the fuselage profile (so as to minimize wetted area drag) is balanced by the creation of separation drag i.e., a "crossover point" exists where we can truncate the profile and not pay a penalty.
Note that several years ago, VW built a prototype ultra-high-mileage car with a teardrop shape- but with a blunt, chopped-off tail. The designers had discovered that extending the tail taper (making the vehicle longer) did not reduce drag enough to be worthwhile, and so they determined where that crossover point was and chopped off everything aft of it.