From Portals of Phereon Wiki

A skill that allows the user to give Mana to another, it can also be used on the Overworld. Since it uses more Mana than is given, it's not an efficient way of restoring Mana.

The Rare Essence Transference allows everyone in your party to use this on the Overworld, which makes managing your party's Mana easier, particularly if you have units that can recover Mana on their own.