Celestial mechanics + understanding of where to put a gate = dropping a meteor moving at 75 km per second on anything you want with a bit of research, for one.
"Knowledge is power" is a trueism, but "power" is more than just 'the ability to kill someone". It could be knowledge of solutions to major problems of economics, culture or law, or medicine. The usual appellation of 'knowledge is power' is 'understanding how the world works can bring about miraculous discoveries or inventions' like modern chemistry... or else make people do what you want because you have dirt on them... or the means to honestly help them with their own problems.
Of course if your players aren't up for you starting the Renaissence or Enlightenment or Industrial Revolution or Scientific Revolution knowledge is a lot less powerful. Innovations and inventions mainly, or else discoveries of resources (remember that who controlled El Dorado, if it were real, held wealth and power unimagined) or peoples to exploit.
To use a fantasy example, in a world dominated by bronze age chariots, recruiting and training a regiment of centaurs in combined infantry and cavalry tactics straight out of Napoleonic europe, creating what amounts to the perfect dragoons, would grant you a stupendous advantage over opposition. They could probably take on forces 10x their size and win, and even beat actual dragoons as they seamlessly switch from cavalry to infantry maneuvers and back without worrying about flagging mounts on top of their own exhaustion.
Knowing how to tame Griffins or Pegasi is also another game changer. Air Superiority means that you literally cannot lose in today's world, merely stall if the ground forces aren't up to the task. How much harder then, in a typical setting, would a typical society fall to regiments of flying armies? If they have air forces, the location of BETTER ones. They field griffin riders and harpies? You throw down some Pixies and Rocs acting as B-52's with escorting fighters.
Knowledge of anesthetic herbology? You could probably convince your DM with little argument that working anesthesia would get paid through the nose, unless he rules that everyone's got a family adept casting cures for them.
Routes to other planes? Opening trade with the City of Brass or Celestia or Dis would definately profit all involved. You could make hefty sums fronting goods to and from there. (in fact in the pre-modern world, trade pretty much WAS a D&D adventure, you sailed or marched across thousands of miles in dozens of lands making friends with or fighting against all manner of strange peoples, encountering wonderous and bizarre animals, and risking your life all the while. People paid insane premiums for distant imports because of how hazardous it was to move goods across the world!)
