I'm no aeronautical engineer, but .. I would think the angle may be
chosen so that, at some chose speed (like final approach speed), there
is minimal trim change over a wide range of power (thrust) settings. I
wouldn't want the airplane to pitch up, or down, on short final when I
found it necessary to add power to make a missed approach.

I would think a similar reason would explain the off-axis alignment of
the motor on single-engine airplanes. I'll ask my mechanic.

george