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Why do most cetaceans have a dorsal fin?

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Illustration showing various cetaceans, including baleen whales like the blue whale, fin whale, humpback whale, gray whale, sei whale, northern right whale, and bowhead whale, as well as toothed whales such as the sperm whale, narwhal, beluga, Baird's beaked whale, killer whale, dolphins, and porpoises. Each species is labeled with its scientific name and typical or maximum length, with a size scale in the corner for reference. — cetacean Whales (order Cetacea).

Most cetaceans possess a dorsal fin primarily to function as a keel, which provides essential stability while swimming.1

In aquatic vertebrates, the dorsal fin plays a critical role in preventing yawing, or lateral seesawing, movements. By extending at right angles to the body, the fin helps the animal maintain balance and control its orientation in the water. For these fins to be most effective, their surface area is typically positioned behind the animal's centre of gravity. Unlike the bony structures found in many other vertebrates, the dorsal fin in cetaceans is composed entirely of connective tissue.21