Priests can be a very important part of a Human army. While Healing and Dispel can be very useful many prefer Priests for their Inner Fire ability. Inner Fire can be an alternative to Orc Shaman Bloodlust and especially useful in team games. Healing can help return units to full health saving you the cost of replacing them. Dispel Magic is useful for countering powerful spells such as Bloodlust and Slow. Dispel Magic is also useful against Skeleton and other summoned units.
Remember to research Adept and Master Training. Besides just giving extra spells these upgrades also increase mana capacity and mana regeneration which is exactly what you want on spell casting units.
Increases a target friendly unit's damage by 10% and armor by 5.
Priest Master Training
Area of Effect
Air, Ground, Friend
Damage +10%, Armor +5
Inner Fire is a very powerful spell since it both increases damage and armor. On Knights it can make them very difficult to kill. Inner Fire is especially useful in team games where many allied units can be buffed with Inner Fire. Inner Fire can be a good alternative to Bloodlust.
Despite the high elves' official departure from the Alliance, some elves still remain true to their former human and dwarven allies. The altruistic priests of Quel'Thalas refused to abandon their roles as healers and agreed to remain in Lordaeron despite the edicts from their reclusive masters in Silvermoon. The high elven priests use their Light-given powers to heal the wounded and bolster the spirits of Lordaeron's fighting elite.