The purpose of setting arm rest is to remove the forearm involvement in moving the stick. With the forearm involved it is difficult to make precise inputs to side stick. With forearm resting it is easier to make the small inputs that are needed for flight path change with only the wrist. Second, any time the stick is out of neutral it's ordering the computers to change something. So after achieving the intended change to the flight path the stick must neutralized. stick free the Airbus maintains 1g path with whatever bank or wings level. It's intermittent contact handling. Pilot handles controls(side stick) only to change the flight path but not to maintain it. That's done by the aircraft automatically. That's the way it is designed. This needs to be emphasized often initially because it differs from non FBW aircraft. Any undesired aircraft movement is pilot induced and answer to that is to leave the stick alone and start again.