In the past Protector was not really in use due to players usually choosing Blood Rage over Protector.
Since patch update 12.55 which allows knight have both- Blood Rage & Protector- it can be useful in critical moments - like when you are about to die or when your druid is in danger of dying and can't heal you as he will be healing himself. It also helps to block more difficult pools with mobs, stronger monsters and bosses.
Protector provides Knight better possibility of survival and fact that Blood Rage can cancel its effect immediately, gives also opportunity to apply appropriate amount of damage on bosses and still have chances on better loot reward.