The ersatz eye from Xanathar's Guide to Everything (p. 137) seems to me as (almost) an entirely cosmetic item.
It would seem to that the loss of an eye is a storytelling option, and the choice to remedy it by using the eye (rather than some sort of healing) is also more of a "rule of cool" thing rather than something that gives a mechanical benefit.
Is there some rule or game mechanic that I'm overlooking that requires that this item need attunement?
Best Answer
The Dungeon Master's Guide contains rules for lingering injuries, including the loss of an eye. If those rules are in use, then this item would provide a way to mitigate one of those results at the cost of an attunement slot.
In a game where the lingering injuries table isn't in use, there's no mechanical benefit to this item and no need for it to require attunement.