The sharping ring is used on paraguayan style harps since there’s not really no place for levers when the harp is traditionally constructed with the split neck design.
Rings or taquitos (protruding stops along the center strip that you can press the string against) were the only way to get sharps and flats. Nowadays many of these harps are built with a sort of extension under the neck to give a space for levers.
The ring doesn’t work very well on most European style harps, even if you wanted to do it that way, because there’s not enough give in the strings. (Paraguayan harps are much more loosely strung, usually.)
But it’s quite tricky to get accurate sharps with the ring, because you must shorten the speaking length of the string *exactly* 1/18th of the original length. Deviate even a little and you won’t get an exact half step.
I think most people find levers a lot more handy than the ring would be, and pedals much easier than levers for making accidentals.
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