What is this airplane used for?
While I'm not sure about the condition of the aircraft, there are uses for a non-flight worthy aircraft in a carrier deck. For example, the crash and salvage team can train salvaging the aircraft, or the sailors can train craning the aircraft abroad the deck.

For example, in the image below, U.S. Navy sailors use a crash and salvage crane to lift a non-functional McDonnell Douglas F/A-18 Hornet during flight deck drills aboard the aircraft carrier USS Ronald Reagan (CVN-76). 



By U.S. Navy photo by Mass Communication Specialist 3rd Class Jonathan Nelson - This Image was released by the United States Navy with the ID 150415-N-WO404-124 (next).This tag does not indicate the copyright status of the attached work. A normal copyright tag is still required. See Commons:Licensing for more information.

It may also be used for firefighting and damage control training, as can be seen in the image below.



Sailors assigned to the crash and salvage crew on USS George H.W. Bush (CVN 77) spray foam on an F/A-18 Hornet training aircraft during a firefighting drill; image from warbirdsnews.com, credited to USN

The article notes:


  Since active jets are precious assets, they use specially-prepared training airframes, like grounded F-18 Hornets ... these former frontline fighters are often antiquated models of current equipment or planes that have been damaged beyond the point of repair. These days the trainers are usually represented by earlier models of the F-18 whose variant has been retired, ... (they serve) as training aids for firefighting and crash retrieval and salvage operations, letting crews hone their skills using real world items, without risking damage to expensive operational flyers