Well it's the same idea all do for drainage. To keep the area open you put in gravel. That way native dirt cannot get in, but water easily can.

Why exactly are you trying to prevent any and all contact with native soil? Is you soil so poorly draining it would imped drainage out of the pot? Arsinic contamination?

I am having a hard time accepting that a few square inches of mix to soil contact on the bottom of a pot would be detrimental to the pH of a pot's mix. Gravitational effects, drainage effect, and limited surface area seem sufficient to prevent significant 'contamination'. The vast majority of the mix would still be in a closed system to prevent latitudinal ion transfer. Plus, you will get better drainage if the mix contacts the soil (presuming it isn't impermeable clay) than if you have it raised on gravel or bricks.

I have experience on an island with limited drainage. We have no water system on the island. No utilities except electricity. No gas, no water. We often have dry wells for our sinks. Even broken glass work if you don't have gravel. It works extremely well. We use a two layer system to run off ground water, a pipe with holes surrounded by gravel then sand. It works so well. The sand keeps roots out of the drain pipes. I dug one up over 35 years old and not one root in the drain line.So using gravel will improve drainage below your pot extremely well. If you want to keep the roots in the pot, put sand on top of the gravel. Then the pot.