The purpose is to have them submit body and soul with [Enslave], while avoiding any unnecessary trouble with the other elves with [Conceal].
Somehow, it seems that the Elves had a culture or custom where cutting a hole in their ears for the attachment of ornaments is a serious taboo—By doing that, it seems their name will be struck from the clan forever, and they will be exiled as punishment; for that reason it seems that Elven slaves that circulate in the markets of this world had their characteristic long ears cut halfway—having these ear-integrated cuffs installed was mostly to try to deny them their last bit of resistance.