The shock towers on a car act as a brace for the bulkheads, so a thicker one allows less bulkhead deflection which in turn makes the car "dig in" harder into the surface of the track. This combined with less flex on the tower itself allows for a car to respond quicker and have a more direct feel on high traction surfaces. Is it a need? Not particularly, it's just a tuning option.
The upper deck is necessary to keep belt tension, increase stiffness and also to prevent the car from flexing too far.