The obvious answer to the question is “To keep from dying while fighting difficult mobs.” But that doesn’t actually answer the underlying question: Why healers at all?
The MMO combat system is usually built on two bars, the red one (health) and the other one (energy, mana, rage, endurance, whatever). The healer trades some of bar two, which we’ll call the blue one from now on, in order to restore some of the red one, either for themselves or for others. Most healers classes in modern MMOs also have the option to dump some of an enemy’s red bar to deplete, but that’s not the focus of this post, so we’ll skip that for now.
The question is, why is this a class? The main reason is that, while buffing and debuffing can be used to somewhat supplant the healer, unless the game is designed in such a way that an enemy can be debuffed to always miss or never cause damage, or, alternately, unless a PC can be buffed to never be hit or never take damage, there’s always going to be the unlucky streaks of damage that cause a PC to die even through buffs and debuffs. Most games aren’t designed that way, simply because the players will trivialize any encounter they can by packing in enough buffers/debuffers to do so. Healers are reactive in their playstyle, being, essentially, Bizarro-DPS. “Me am heal you, you am never die!” Even the most Heal Over Time style of healer still needs to pay attention to health bars and react to sudden spikes of damage.
However, what if we simply changed the placement of healing abilities? Let’s assume that it’s a given that damage spikes need to be dealt with, that we don’t want players dying simply because of bad luck. So let’s offload the healing from a specific class to everyone. Maybe with different degrees of skill and effect, but everyone, so long as they can get a brief respite from being wailed on, can at least take a knee in combat to collect themselves then get back into the fight. Maybe one or more of the classes still retains healing spells, but they are strictly post-combat affairs to reduce downtime or absolute emergency in-combat things, like the WoW Paladin’s once-an-hour full heal.
How does this change the party dynamic? Well, it reduces the number of must-have characters, for one. No serious dungeoneering is likely to be done in most current MMOs without a healer along. The tanks can’t heal themselves, which means they can’t keep the DPS safe, hence the trinity. However, if everyone has the ability to heal themselves a bit, enough to survive an averagely-handled encounter, then there’s no reason to need a dedicated healer.
It changes combat dynamics, too. Having two or even three tanks suddenly becomes a good idea, beyond just having them handle main tanking and emergency- or off-tanking. As an idea, let’s say warriors have two heals, one that cycles in half a minute, heals them for half but lets them stay in the fight, and another that takes a minute to recycle, requires them to not get hit during its 5 second cast, and heals them to full. It’s possible to tune the fights so that, most of the time, that one tank can get things done. However, if there’s an emergency, like the warrior pops his half-heal, then immediately takes a heavy crit, the other tank needs to do his best to grab aggro for a bit while the warrior withdraws from any AoE radius, then goes and rests for a few second. On the other hand, if there’s only the one tank, the DPS suddenly needs to step things way up and either briefly act as off tanks, which they’ll be better able to do with heals of their own, or simply burn down the mob before it kills the tank. Lastly, if there’s no tanks at all, the DPS can try to act as a round-robin series of temp tanks, popping their relatively weak/long-recharging self heals or using powers like mana shield, evasion and power word: shield to survive for very short stretches of tanking.
It also helps to end the “rock star tank” problem where there’s 4 healers all firing salvoes of heals off while the tank keeps spamming aggro grabbing abilities. Dangerous encounters become a complicated dance of multiple tanking and DPS instead of its current “airplane pilot” status: solid stretches of boring button pressing, interspersed with sheer terror as one of the DPS dealers suddenly overaggroes. In addition, all of the best warrior/paladin gear goes to one character in the guild, because, well, he’s the main tank and the sole focus of aggro most of the time. There’s already some tank swapping in existing games, particularly some of the more complicated WoW raids, so we know players can handle it.
Credit where credit is due, the new 4th edition D&D design notes are what got me going in this direction. According to their preview stuff, they’re expecting a cleric to make spend one action per combat healing, with the rest fighting and buffing. Clerics in that will still be the masters of post-fight recovery, but by moving around heals so that everyone has at least a limited self-heal, the designers are looking to make it so that everyone is constantly making an interesting choice in combat, an idea that I think MMO designers would do well to borrow.