The conventional explanation for cormorants’ habit of spreading out their wings is to dry them. Cormorants’ preen glands, apparently, do not have enough balm, or it is not effective enough, to coat their feathers with waterproofing in the way that virtually every other diving bird enjoys. Consequently, the cormorant’s wings get wet during their dives, and they have to haul out and dry them off. That makes sense. However, some species of cormorants that do not go into the water have the same habit, and some species that dive regularly don’t. According to the Wikipedia writers, there are competing explanations:
After fishing, cormorants go ashore, and are frequently seen holding their wings out in the sun. All cormorants have preen gland secretions that are used ostensibly to keep the feathers waterproof. Some sources state that cormorants have waterproof feathers while others say that they have water-permeable feathers. Still others suggest that the outer plumage absorbs water but does not permit it to penetrate the layer of air next to the skin. The wing drying action is seen even in the flightless cormorant but not in the Antarctic shags or red-legged cormorants. Alternate functions suggested for the spread-wing posture include that it aids thermoregulation or digestion, balances the bird, or indicates presence of fish. A detailed study of the great cormorant concludes that it is without doubt to dry the plumage.https://en.wikipedia.org/wiki/Cormorant
To my mind, the more interesting issue here has to do with flight. As is well known, a bird could not fly if it captured as much air with the upstroke of its wings as with its downstroke. Birds make quick and subtle adjustments to shed air on the upstroke and then catch it on the downstroke so as to give themselves lift. It’s fascinating and complicated but that’s the basics of it. Now, look at this cormorant perched on its stone. It beats its wings furiously, yet it doesn’t fly. At its first effort, it almost fell off the stone, but it quickly caught itself. How does it manage to flap its wings without taking off? It must have a way of shedding air both on the upstroke and the downstroke. Possibly it spreads its flight feathers far apart as it flaps so that the air passes through its wings without exerting leverage on the bird’s body.
Cormorants are ancient and storied birds. It’s well worth reading the Wikipedia article on them, even though the piece repeatedly strays into the weeds. The evolutionary ancestors of the cormorant, very similar to the modern bird, were alive at the time of the dinosaurs.