There are a variety of reasons for getting a factory unit with a lower horsepower rating than the normal model. For example, in switching duty, the locomotive spends a lot of time at idle and only short periods working hard. Starting tractive effort, which is what is important to move a cut of cars in switching duty, is pretty independent of prime mover horsepower, but is a function of weight on drivers, gear ratio and the electrical rating of traction moters and the main generator.
Suppose a railroad wants a switcher with capability to occasionally haul a short train at over-the road speeds. An SW model might work but it only has switcher, not road trucks. This sounds like a perfect use for a derated GP9M with only 1350 horsepower. The locomotive will have the same starting tractive effort of the standard model for switching duty and it will have the Blomberg road trucks for good ride and tracking on the occasional over-the-road movement. Of course it wouldn’t be able to handle as long a train, but that might suit the need just fine. So, the question for this railroad would be, how much horsepower SHOULD this new locomotive have? The answer, based on the history of the required operating conditions, might be 1,350 horsepower.
Remember, too, that any locomotive is burning fuel when at idle. For switching duty, this could be a substantial part of the operating cost. So, why not get a smaller prime mover to reduce fuel costs?
Another possible scenario from the past would be the need for a switcher with steam generator capability to handle moving passenger equipment in the terminal. Again, a standard SW unit has no steam generator, but a GP model has room for a steam generator and capacity for a water tank on the undercarriage to feed the generator. I believe there are still a few such units in Amtrak service, but they are rapidly disappearing and may be gone by now. Certainly the need for steam is gone.
I’m sure there are other reasons for odd, low horsepower units