I do not believe that you are increasing the spring rate. Because you are not pushing up against anything but the vehicle's weight. Until which point that you reach the stops of the suspensions travel. You are only raising the "at rest position" in between the upper and lower limits of the suspension's travel. Your spring's "preload" does not change as the spring is still under the same amount of load.
To change the spring rate would require swapping out the spring for one with a higher spring rate. Lbs. per inch. "a stiffer spring"
You are just raising the bottom of the spring, with no additional load to the top of the spring. Only the static position of the geometry is changed.
Think of the travel of the suspension between the upper and lower limits as your suspensions "stroke".
Through out the stroke the angels of the geometry become more and more severe as you get closer to these limits.
Therefore, to reduce these angles severity in the geometry you want to have you static point closer to the middle than not. Not just to avoid hitting the limits of the stroke.
