Well, in ages long since past, when the tanks were still more different from each other and tank stances had other effects than just enmity...

They all reduced damage done, but gave other bonuses... Both PLD and DRK had as a part of Shield Oath and Grit reduction on damage taken, meanwhile WAR's Defiance gave them more max HP and healing received.

And for when not taking, PLD had Sword Oath, which essentially just increased auto-attack damage by 50%... And WAR got Deliverance which increased all damage done by 5%.