I am copying and pasting what Brenner wrote in his contra thread. It is clear like water:

1/.. P-factor from prop blades having unequal angles of attack is cancelled out.
2/.. Motor torque into the airframe is divided by the gearbox gear ratio.
3/.. Gyroscopic moment is cancelled out
4/.. Spiral airflow over the wings, stab, & rudder is straightened.
5/.. Propeller efficiency is increased by 10%
6/.. It makes a funny noise when the plane flies. (not loud, just funny...)
7/.. It's much harder to stop the props from digging into the grass when you drag your plane back to the pits.

In the first page of the contra manual there is more detail. I could scan tomorrow is someone is interested.