Giving fish its silky, smooth texture when raw, and a flaky, light texture when cooked.

Sushi's Secret: Why We Get Hooked On Raw Fish : The Salt : NPR:

One fish, two fish, white fish, red fish: Muscles that depend on oxygen tend to be red, while those that don't are white. Salmon flesh is orange because of the food the fish eat.
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But why do so many of us find utter bliss in eating raw sea creatures but aren't so inclined to chow down on uncooked birds, cows or pigs?
A big part of it is gravity — or the effective lack of it in the ocean, says biophysicist Ole Mouritsen, author of Sushi: Food for the Eye, the Body and the Soul.
"Fish are so soft. You can stick your finger through their muscles," he says. "Try doing that with a chicken or cow. Fish muscle is very different than that in land animals."
Why? Because fish can afford to be lazier than terrestrial animals. Fish essentially float all the time. So their muscles don't work constantly to fight gravity.
"Fish don't have to support their body weight," Mouritsen says, "so their muscle fibers are shorter and less tough than those in land animals." The same goes for the connective tissue holding the muscle fibers together: It's delicate and weak.
The result? Fish has a silky, smooth texture when it's raw, and a flaky, light texture when it's cooked.