As it is impossible to sail the sea around the Land, due to the Ring of Fire (an everlasting thunderstorm), most transport happens through the air. This is even more necessary as the capital is built on several rocks, connected by bridges. One even needs a Plunder or another flying animal to be able to enter the city at all. Although there are other flying animals, like an animal that looks like a giant bird/fish hybrid, Plunders appear to be the standard used animals for flying.
While we regularly see Plunders flying in the air in the story, there are three moments in which the Plunders play an active role. First, Ember is saved by a Plunder, controlled by Diamet when she was forced to jump out of the old sewer and into the abyss when Prowesse had poured burning oil into the sewer. Later on in the story, when Storm and Nomad escapes from Prowesse's dungeon, they jump down from the rock and land on a laundry carrying Plunder, after which Nomad asks the pilot to bring them to the Royal Palace. And at the end of the story, Storm, Ember and Nomad have to flee the Land inside the Ring of Fire on the back of Plunders.