Look, almost all the aircraft you will see a small hole behind - this nozzle APU. Why not put some engine easier or a bunch of batteries and starter motor in the car?... Now it becomes clear purpose of these hoses and this [van]. The auxiliary power unit may be damaged, or you may need to provide the voltage or quickly warm up the plane after a long winter parking. In these and other cases, the [van] with a gas turbine engine in the cabin to help.