From the Super Mario Wiki
The Biplane Barrel is a special biplane-shaped Barrel that is owned by Funky Kong in Donkey Kong Country 2: Diddy's Kong Quest and Donkey Kong Land 2. It serves as the replacement for the original Jumbo Barrel from the first Donkey Kong Country.
If Diddy or Dixie Kong pay a visit to one of the Funky's Flights II on Crocodile Isle, Funky will let them ride around to another area of Crocodile Isle, provided that they've already been there, as the only other method of returning to the main Crocodile Isle map is by defeating a boss. This is done as a means to prevent level-skipping, but serves as a safe alternative to having to fight a boss.
While usage of the Biplane Barrel will cost two Banana Coins for the first ride from every Funky's Flights II location on Crocodile Isle, all consecutive uses from any location will be free.
In DK: Jungle Climber, the plane that is used to travel to the game's various islands largely resembles the Biplane Barrel in appearance. This plane needs to be fueled with Oil Barrels in order to be used.