Because you want to replicate load and know for that load range exactly what ignition timing and carb settings make the most power and power width.
From data we know that kart acceleration is not proportional to gearing. In quite a wide range, you can go taller on gear and not lose any significant low end accel, yet gain massive speed advantage up high. So we know that not just engine speed but engine load, and/or carb sensitivity to engine acceleration (engine speed derivative), are key factors in kart engine power width (at least for KF2) .
Walt, thanks.. when you change gears on an inertia dyno and overlay on-dyno engine speed vs on-track engine speed, do they line up perfectly? Do you mind showing some examples? Really appreciate it.
I tend to think the on-dyno engine speed trace will be closer to linear vs the on-track one mainly because of air drag increasing as a sqaure of speed, but perhaps in karts and at typical kart track ground speeds, it is small enough not to matter.