Never really gave it much thought and Google isn't helping give any reasonable answers.
Free (neutral) revving versus under-load revving.

I had to do the California SMOG check the other day and they put the car on a dyno to do NoX testing.

Other than speed sensors feedback to make the ECU happy,
WHY does it matter if there was a dyno in place or not?
I guess the same goes with tuning - why does the engine get put under load?

When the engine is turning 4000 rpm in neutral, isn't it going to put out the same exhaust gasses as 4000 in gear?

 

It needs to be loaded to simulate real world driving, Think of it this way if your driving on a flat road at 60mph and then you get to a hill and maintain that 60mph you have to depress the gas pedal further. You do this because there effectively needs to be a larger combustion even (read more power) to maintain that speed when the engine has to work harder to get up the hill. Depressing the gas pedal increases air volume which is then calculated and fuel is injected at an increased rate to match. If you were free revving the car the engine does not require the same level of power to spin just the crank and rods/pistons at 4000 rpm as it does when you are also having to pull/push the vehicle down the road. Hopefully I have cleared this up for you if not let me know and Ill try to explain it better.

 

No. Would you burn more calories pulling a 10 lb. wagon down the street or pulling a 2500+ lb. car down the street? Same laws of physics apply to the engine; it takes less energy to spin a 10 lb. flywheel than it does to pull a 2500+ lb. car.

For you, more energy = more calories burned.
For the engine, more energy = more air & fuel burned.

Nothing is free in physics. Moving more mass requires more energy and there's no way around that.