The lighter that we can build safe aircraft the better. Reducing the operating empty weight of commercial aircraft can allow for an increase in the passengers, baggage, and cargo that the plane can safely transport. Early aircraft were made of wood and fabric. An example of an early aircraft is shown in the figure below. This provided a good combination of lightness and strength but required reinforcing struts, which added weight and drag and resulted in multiple wing designs.
Improved airplane engine designs resulted in more powerful engines and higher airspeeds. As speed increases, drag increases nonlinearly. Single wing (less drag) airplane designs were required to take advantage of the improvements in speed.