Contaminated fuel on the ground can lead to serious problems in the air. Here's an example of pilots that sumped their fuel and still had a contamination-related engine failure.
Teaching complex maneuvers will test your knowledge and skill as an instructor. Sometimes setting up the maneuver is even more complicated than the maneuver itself.
It's pretty much impossible to explain aerodynamics without heavily simplifying it. Aerodynamics is a field for engineers, based on differential equations that don't have much use in the cockpit.