Why become a Priest?
For some, the answer may be as simple as they played a cleric in other games and feel that's their calling. For the rest of the world though, there are essentially four healing classes - Paladins, Shamans, Druids, and Priests. Paladins are the melee-centric healers and wear plate. Shamans and druids are closer to being hybrid in their non-healing capacities and wear leather.
Priests are what I like to call the "caster healers." They wear cloth, their weapon choices are limited but include the use of wands, their mana grossly outweighs their HP in raw numbers.
Technically, other classes have more mana efficient heals, but the priests are stronger, and the priest has an additional tool that they don't called Holy Word: Shield. Shield, which some people call bubble, is the staple spell for the priest, as it prevents all damage up to a preset amount that it is set to absorb. This acts as a sort of a pre-heal, but it also lets the priest cast on herself to be able to cast spells uninterrupted, and as an instant cast, to buy the time to heal someone who is nearly dead.
Even if you aren't into playing a cleric/white mage group healer-type, the priest is still an enjoyable class, and enjoys one of the best soloing capabilities in the game right now. While not the absolute most offensive class in the game, the priest can put out some decent damage, especially over the short run. Besides damage spells, the priest also can use wands, which are ranged weapons that have no ammunition to fire, and since they are magic-based, tend to put out some very nice damage. You can "melee" fairly effectively with a wand, albeit you have no special skills that a rogue or warrior would to augment the damage.
In addition, the priest has a number of utility spells that can be very handy. A self-only mem blur. A lull. An AE fear spell for getting 3 creatures to leave you alone for a few seconds. A mana drain spell (that doubles as a nuke). At level 30, they get Mind Control, which enables them to charm humanoid creatures for up to a minute. They also get a few buff spells, most notable of which is an indirect HP buff.
The priest is a fun class to play, powerful to solo with, and well-appreciated in a group setting. Anyone who says the priest is nothing more than a "bubble healer" is dead wrong. The priest is a powerhouse class that surprisingly few people play.
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Only the exceptions can be exceptional.