Aircraft pilot's seats: what is the notch for?
It's either for clearance of the column of the yoke or moving the crotch strap of the harness closer to the body of the pilot.

Alternative yoke designs remove the need for the notch:


(source: airliners.net)  

Alternative control methods like the Airbus sidestick also remove the need. As Henning Makholm noted, the seat strap just goes through the cushion.