Penguins have evolved from birds that used to fly. At some stage, the penguin ancestors became increasingly adept at swimming and lost the ability to fly. In the trade-off, their short, powerful wings make excellent flippers for "flying" underwater, being less fragile and creating less drag than the long wings of many other seabirds. Another advantage of giving up flight is that they can put on weight. The ability to store food reserves as fat is an advantage for birds that have to travel far to obtain food, and larger penguins can dive deeper and for longer.
In the evolution of penguins, what advantage did they gain by developing short, powerful wings suitable for underwater "flight" instead of the ability to fly in the air?