the difference is putting your car on a lift and measuring the power at the wheels. versus putting your car on the street and GA 400 telling you how much power it feels from your wheels.

no load versus a true simulation with weather factors/ vehicle weight/ vehicle aerodynamics all taken into consideration. the faster your car is going on the street, the more these factors play a part in how fast you can accelerate and how fast you can go. therefore when you're tuning your car for how it's going to be on the street, wouldn't it be easier to calibrate your car (tune) when you can simulate the exact same conditions on a dyno? -rhetorical-

Dave