The fuselage of the aircraft connects all components and provides the necessary transport capacity. In the fuselage of the aircraft sit the pilot(s), the passengers, and the load to be carried is primarily accommodated here.
The shape of the fuselage contributes significantly to aerodynamics, and slim, lightweight construction is the rule. Since in the other assemblies of an airplane there is usually no space for the many devices, computers, hydraulic system, etc. as well as often the fuel tanks are integrated in the fuselage.