Temple Priest is the cheap unit of the Vanguards. In an expensive deck that needs a strong board presence, this 0-cost unit allows you to keep the action going while saving up magic for a champion.
With “Guardian Spirit” and/or “Holy Judgment” active is when they are at their best.
Once you have your board set up, your priest can shift from combat units to healer of champions. They heal an average of 2.33 life per card, which is not bad if you have the extra attacks going. Healing Citadel Knights is not worth it in general and Paladins barely so, but if you have the free attacks, and it can make those units survive one more attack, it is worth doing.
Remember, “Holy Judgment” buffs their healing capability. In that situation, their heal becomes a lot more attractive of an option.