S shaped arms allow for a greater pivot point distance than a straight arm - so less tracking error....The pivot to stylus distance determines the degree of tracking error, and since the pivot to stylus distance is a straight line, different shapes of tonearms with the same pivot to stylus distance will trace the same arc.
The greatest advantage to a S or J shaped tonearm is it ability to be designed with a detachable headshell as the offset angle is built into the arm design