When tightening lug nuts, after changing a tire... (2 questions)?

I have always heard you tighten the lug nuts by hand while the car is in the air, slightly lower the car so that the tire is resting on the ground (though not the full weight of the car) and then use the iron to fully tighten them. Why not fully tighten in the air? Why not fully tighten after completely lowering... show more I have always heard you tighten the lug nuts by hand while the car is in the air, slightly lower the car so that the tire is resting on the ground (though not the full weight of the car) and then use the iron to fully tighten them.

Why not fully tighten in the air?
Why not fully tighten after completely lowering the car?
Update: I would like to place emphasis on the second question:

Why not fully tighten after completely lowering the car?
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