What is the point of gravel in the bottom of a post hole?

What is the point of gravel in the bottom of a post hole?
Is it to keep the post from sitting in water?
Say the hole is 24" deep with 2" of gravel. When the water line is 22.2" below the surface, the gravel is almost covered with water and the post is dry.
Buy when the water line is 21" below the surface, the gravel and 1" of post are under water.
Why not just make the hole 22 inches, and skip the 2" of gravel?
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The water table moves up and down.
--
Dan Espen

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Don't the spaces in the gravel fill with dirt within a year or two? And woudln't it wick almost as well then, because once the water got to the post it woudl be sucked into the post and more water would arrive soon?
At any rate, this makes my failure to check for gravel before putting in the replacement post even worse. This post lasted 35 years, and only rotted at ground level and about 6" below ground level. The bottom 15 or 18 inches of the post were still perfect. And the other 38 posts still sturdy and firmly in their holes. (It might be another 10 years before another post fails and I can check for gravel. )
Is it likely they didn't use cement or gravel in my yardl because the dirt is almost all clay?

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