My guess? If we figure out how to detect dark matter we can get closer to figuring out how to interact with it (other than through the very very very weak gravitational force). Or maybe we figure out that it was a spinning universal frame or something that gives us a better Standard Model.
If, however, we figure out how to interact with it and can harness any potential energy from it, then by definition we won't see any interference in the electromagnetic forces. That would be incredible, that would be as good as having readily available superconductors.
It was only years after his death that Parsons would use his work to determine the necessary geometry of the rotors and stators in his new and revolutionary engine. Regnault certainly didn't see that coming, the people who overlooked his work didn't either, but that pure science for the sake of science helped to change the world.
So as you say, we often don't realize what we're going to do with the results of pure science until engineering catches up, sometimes decades or centuries later. Still without the pure science we'd never get the engineering.