So here is a real life experience a friend of mine had on the Spiegel Grove in Florida.
They had finished their dives and were packing up their gear when the DM on their boat started shouting. He was yelling to the crew of another nearby boat and to their passenger, who was struggling to get to the boat on the surface after his dive. He had not properly inflated his BCD. The DM tried to get him to inflate the BCD, and he tried to get the other boat's crew to pay attention. Neither paid any attention to him.
Finally, he pulled on his own fins, mask and snorkel, jumped in, and swam rapidly toward the struggling diver. The diver sank below the water before he got there. The DM got to the spot and dived down, by his estimate catching the rapidly sinking diver at about 20 feet down. He pulled him to the surface and got him to his boat, where CPR miraculously revived him.
That's why you need to be able to do that sort of thing.