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 and applied loads. It 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 conform to the available space. Payload and flight systems are often contained in a fuselage that is essentially a long cylinder. Gliders only need internal volume for the pilot, so they often have a pod cockpit and a boom that provides a long moment arm for the tail. For the Mars airplane, the packaging task is to find a fuselage shape that has a sufficient internal volume for the payload and flight systems. The payload must be positioned so that the instruments and cameras can operate properly.