But WHY??

I knew you were going to ask that. In order to illustrate the origin of the burlesque term "third banana", I present this clip from the soundtrack of Ignorance Is Blitz (1942) featuring the team of Soupy Farnsworth and Bobby Katz, assisted by the worst straightman in the history of the universe, Milton "Sixth Wheel" MacMillan. In the film, this ancient burlesque routine, the origins of which which may predate animal life, takes place in a trench in the middle of a battlefield. In the following scene, Soupy gets his legs blown off by a shell with hilarious consequences.