Simulators teach systems and procedures
My first encounter with a simulator was during my Instrument Rating. After I got cocky with my sim approaches and holds, the airplane rapidly readjusted my attitude and it took considerable a/c time to learn how to do in the air what was a doodle in the sim.
Later I spent considerable time in a 2-axis Viscount sim that flew like a truck. Very handy for finding your way around a complex cockpit.
The 9/11 miscreants followed a logical path:[list=1][*]Get a PPL[*]Get enough full-motion sim time to find your way around the cockpit[/list=1]There's a big difference between a monitor and an actual cockpit.