Why some small piston airplanes, like Cessna 152/172, Tecnam P2008 and many more, have the vertical stabilizer with a sweepback? I don't think that those airplanes may encounter compressibility effects, so what is the reason?
Two benefits to fin sweep on a type like a 152 or 172:
Marketing, and aerodyanamics.
The airplane looks "cooler", "going fast when it's tied down" with the sharply swept fin; this makes it easier to sell to casual pilots and those who'll rent the plane to casual pilots.
However, there's also another advantage. The sweep moves the tail area to the rear without adding dead weight of structure far out in the tail. Thus rudder authority and yaw stability are gained with little increase in overall weight, compared to the previous generations of the same models in the Cessna line, for instance (look at a 180 from 1960 for comparison to a modern 182).