KPI is important on any front end. While increasing KPI can decrease scrub, if you imagine a front end with no caster and no KPI, it wouldn't matter how much scrub you had, you couldn't lift your inside back.
If you imagine an axle with no caster, and only KPI you will note that the tire is lowest while it is facing directly forward. Turning the wheel either way causes the tire to drop. This helps recover some of the directional instability that you lose by going to toe out. The mower weight pushing down causes the wheels to center and go directly forward.
Lastly we add caster to our axles to make the inside tire drop MORE than the outside wheel while turning to give us the lift on the inside rear.
Finally scrub simply magnifies the amount that caster and kingpin angles lift. More scrub, more lift, but you do pay with the ease of steering. To much scrub and the mower can get very difficult to steer, especially while stopped. Basically we are looking for a nice medium of the big 3 (KPI, Caster, scrub) where none of them are too radical.
Thats how I understand it at least!!