[RayChuang88]

Easy answer: they are used as flying engine test platforms. The upper picture is a plane that belongs to GE Aero Engines, and the bottom one is a plane belonging to Rolls-Royce.

However, it should be noted that Airbus has "loaned" one of their A380 prototypes to Rolls-Royce for use as an engine testbed. The Rolls-Royce Trent XWB engine used on the Airbus A350XWB airliner had its test flights mounted on one of the engine pylons of an A380 test plane.

[Paleo Conservative]

Easy answer: they are used as flying engine test platforms. The upper picture is a plane that belongs to GE Aero Engines, and the bottom one is a plane belonging to Rolls-Royce.

But did you notice the really odd thing about the GE 747-121 testbed? It has 3 P&W JT9D engines not GE CF6's. The plane was owned by GE Capital when Pan Am went into bankruptcy.