Looking at the slightly wider picture, by placing your elbow or forearm on a fixed surface, armrest, you are removing a mechanical link, your upper arm, from the control system linkage. This results in it being easier to make small movements with a finer degree of control and precision.
For those of us who came into aviation via sporting light aircraft, especially gliders, we were taught on conversion to an aircraft with a cranked (bent) control stick (think libelle or astir type gliders) without a parallelogram type linkage (eg kestrel) then NOT resting your forearm on your thigh would normally result in PIO (pilot induced oscillations). In the event of the aircraft pitching nose up, the inertia in your forearm would cause the stick to move back, resulting in further pitch up. Pushing forward, the nose goes down, you unload the force on your arm, and you pitch even more down than you expected. In the past this has resulted in heavy damage and even injury. This is the reason I believe Airbooos insist on using the armrest.