Its just the counterpart to Paladins Blessing of Sanctuary.
They could Buff the whole Raid before with 3% damage reduction, and were the only one that could to that.
And since Blizzard is spreading Buffs around (Replenishment, Scorch etc.) they did the same with Blessing of Sanctuary.
So it will affect the whole raid, once you cast a PW:S, theyll get the buff.