Cornerweighting is not about getting all wheels on the same plane. The ground is rarely level, and when the car is cornering, all 4 wheels are definitely not going to maintain the same contact with the ground.

The goal of cornerweighting a car is to get a 50/50 diagonal cross weight, that is LF to RR. The ensures that the car behaves consistently during left and right corners. From the factory, the weight distribution will be different at each corner most likely, and the only way to adjust the load on each corner is to raise or lower the car, and the process of adjusting those heights is cornerweighting.

 

...which is doing exactly what I described. The L/R and F/R weight distribution won't be changed by changing ride height. You're equalizing the cross weight while trying to get everything at the ride height you want. Its exactly like shimming a wobbly table. The plane thing is just an analogy. If your cross weights are messed up it means that your car is trying to put one wheel lower than the other three (or higher, depending on which wheel you pick).