[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.

[NorthMountain]

OK ... Some more slightly beyond usual 747 pictures. <.P
WTF? Another engine test, I suppose ...

Airborne LASER.

SOFIA infrared telescope

Parts hauler for the 787 program

[RayChuang88]

I think because it was an ex-Pan Am plane, GE Aero Engines decided NOT to replace the original JT9D engines because it would have required replacing a lot of hydraulic, fuel delivery and electrical systems to accommodate the CF6-50 engine. I believe that plane is still active, and will soon have a GE9X prototype engine mounted to test the engine for the upcoming Boeing 777-8 and 777-9 airliners.