Ding, Ding, Ding, Ding...Sonoace Has the correct answer!!!  :D

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It's primary reason is so the FAA can give the pilot a checkride. Some planes/airlines have only one jump seat, others have two. Although restricted in the USA to certain people, and much further restricted after 911, it remains available to most active pilots or others who appear on the individual airline approved list...which is of course approved by the FAA.  :(

In other countries, the jump seat was left up to the Captain's discretion as to who could sit there. It was sometime used as a reward to certain loyal passengers. At TWA for instance, all our planes - except the DC-9/MD80 had two jump seats.

Lou