A great cormorant's plumage is not uniformly water-repellent. The outer feathers wet readily while an inner layer stays comparatively dry, so the bird retains insulation but loses much of the trapped air that would otherwise make it float high. Less buoyancy means less work to reach depth and to stay there while chasing fish.
The cost is paid afterwards. A soaked cormorant carries extra mass and loses heat to the water faster, so it hauls out and holds its wings open. The wing-spreading posture is generally interpreted as drying, and it is most often seen after a bout of diving rather than at random through the day.
Two alternative explanations have been proposed and are worth naming: that the posture aids thermoregulation, and that it helps the bird handle or digest a large fish. Neither has displaced drying as the leading account, but the evidence is not conclusive enough to declare the question closed. What is clearly wrong is the folk claim that cormorants lack a preen gland altogether; they have one.