Toyota designed our headlight circuits so that all the headlight current goes through the high/low beam switch in the steering column which is in the ground path for the headlights. Over time, switching from low to high and back will cause some pitting in the switch contacts which is seen as black discoloration as well as the little divets. It's caused by arcing. Once the initial surface is breached, even though you clean and polish it, the pitting returns pretty quickly. I think it is because the surface of the contacts is hardened a little. Once the contacts get pitted and then you sand and polish them smooth, the underlying softer material is exposed. This softer material doesn't stand up to switching that much current and the pitting returns rather quickly.
With my harness installed, headlight current is no longer going through any of the vehicle wires, switches, or relays. The headlights are now connected directly to the battery through the relays in my harness. The new harness uses the relays in the harness to switch the current to the headlights, so no headlight current goes through the high/low switch or the vehicle headlight relay any more.
The high/low switch and the headlight on/off switch which controls the vehicle headlight relay are still important, because they control the harness functions, but the amount of current that the high/low switch and the vehicle headlight relay are now switching is very small. So, to make a long story short, your high/low switch contacts will no longer experience any pitting because the headlight harness is now switching the power to the headlights.
Make sense?