The premise is that the ring is supposed to go off when a fertile mate is nearby the wearer, and thus a match can happen. I believe its mentioned somewhere in the book that it was used for that particular purpose.

In reality it is merely a plot device, because one cannot have sex with Merry without being in love with them, so if you get zapped by the ring, you are now instantly in love with Merry, or will be soon. I think there is only one or two men around Merry right now that the ring didn't go off for.