It is a matter of practicality.
On a pen that works, the ink is already at the tip of the tines, whether you choose to let it on the paper using the underside or topside, it will write either way.
But on most nibs, writing with the topside will yield firmer and finer lines than otherwise. And sometimes this can be used to our advantage.
I do admit that I feel strange when using the nib reversed for the whole writing (or sketching) session, it feels like I'm not using the pen correctly, no matter how my mind tried to rationalize it.
I also remember a few occasion when I irritatingly discover a nib which writes in an exquisite fine italic when used in reverse, but when used normally, it produce boring and blobby lines.
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