Notice the props spinning toward each other on each side.
I expect that is done to cancel the effects of engine torque which if they were all rotating in the same direction would tend to roll the aircraft longitudinally requiring constant correction.
Its the same reason helicopters have tail rotors, although the torque reaction is different from a fixed wing craft. If you lost the tail rotor the fuselage would try to rotate in the opposite direction of the main rotor. Big cargo copters with twin rotors counter-rotate and so can operate w/out tail rotors.
Regards,
GtG