The bumps channel the water flow and create turbulence. This “tubercle effect” provides the whale with more lift, allowing it to tilt its flippers at a high angle without stalling. At high angles these bumps also reduce drag —an important benefit for the humpback’s long flippers, each being about one third of the whale’s body length.

https://www.jw.org/en/library/magazines/g201306/flipper-of-humpback-whale/