Most modern aircraft use a form of stressed skin structure known as monocoque or semi-monocoque construction.
The monocoque design uses the skin to support almost all imposed loads.
This structure can be strong, but cannot withstand any dents or deformation of the surface.
This characteristic is easily demonstrated by a thin aluminum beverage can.
You can exert a large force to the ends of the can without causing any damage, however if the side of the can is dented, the can will collapse easily.
The semi-monocoque system uses a substructure riveted to the airplanes skin to maintain the shape of the airframe and increase its strength.