A useful aircraft must carry payload, which might be passengers or cargo. For the Mars airplane, the payload will include scientific instruments and cameras. These must be packaged along with its engine, fuel and flight control system. The payload must be protected from vibration. The payload must not interfere with the operation of the aircraft.
Wings are often too thin to provide useful volume for packaging large components.
Commercial transport aircraft use their wings to carry liquid fuel, because the tanks can fit easily into the shape of the wing. Payload and flight systems are often placed in a fuselage that is typically a long cylinder. This cylinder shape gives a good combination of high volume, low structural weight and low aerodynamic drag. For the Mars airplane, the packaging task is to find a fuselage shape that has enough room inside for the payload and flight systems. The payload must be placed so that the instruments and cameras can operate properly.