Back when I did my DM work, it had been many years since I had done a lot of surface swimming (I really don't enjoy it at all), so I was one of those who did a lot of swimming practice to get ready for the tests. When I did the timed tests, I accumulated enough points on everything before doing the tired diver tow that I did not need any real time on that event--I just had to do it.
I was with the dive shop on a trip to Key Largo, and we were about to do a dive on which I had some assigned DMC tasks. I asked the instructor if I could just go in the water with someone, do a tow out and back for an estimated proper distance, and get it over with. He said sure, so off I went.
The seas were a bit high, and I started the swim into the wind/current. I went well past the required distance out just to be sure. There was another dive boat moored nearby, and I imagined myself pulling a real victim to it. It was very hard work, and I was getting quite winded. When I got my "victim" back to the boat so we could begin the actual dive, I could not submerge for a long time. I was breathing so hard my lungs were too full of air for me to submerge.
That experience told me very clearly why that swimming requirement is so very important. I realized while towing that guy against the wind that it might be a very, very hard job to save someone who is in trouble, and I could not be content with a mere technical pass on that requirement. Someone's life might depend on my being able to do more than a dog paddle to save them.
On a later shop trip to Key Largo I did not attend, a man from a neighboring boat was struggling in the water trying to get back to his boat after ascending from the Spiegel Grove. People were shouting at him to inflate his BCD, but he did not. He sank beneath the waves. The DM from our boat leaped into the water, swam furiously toward him, got a hold of him as he was about to get beyond his grasp, brought him back to the boat, and began CPR. The guy revived.
I think the guy was very glad that the DM had strong surface swimming skills.