lowering tire pressure gives a bigger footprint for better traction and allows the tires to absorb some of the bumps instead of transmittiing them to you.
Too little air can result in a damaged rim if a solid object is hit at speed.
I question the need to go to extremes on bikes as heavy as the F800 or 1200 bikes. In the first place, riders are really less likely to ride them in extreme conditions and secondly the heavy bikes are already applying more downward force on the tires, compared to a light bike.
I do not know just how strong your rims are, but I have dented my F800gs rims running at 22lbs and now keep them above 28lb because of it.
What works is going to depend on the terrain and how fast you ride.
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