A healer is a pirate in a crew who is delegated to attune their crewmates with a voodoo doll, and keep them healthy by using their Heal and Cure hexes, or with a voodoo staff that has Nature Aura during a battle. Also, they may be the designated person to Revive a fallen mate. Reviving a pirate costs 1 Health Tonic.
Healers tend to be higher-level pirates who can withstand numerous enemy attacks, while the others fight without fear of being defeated easily. Or lower level crewmen who need to stay clear of the fighting while helping their teammates.
Before a major fight, like boarding flagships or entering a dangerous area, a healer can quickly attune the members of the crew who are the most vulnerable.
As a healer, should the target of their healing kill an enemy, they actually gain a Reputation reward depending on how much they healed the other player.
They will need to attack the enemy at least once to be able to open the loot drop after the enemy is defeated.
Most NPCs have the ability to heal you for 5 gold, if you are missing health, making them useful when traveling alone or during Jolly Roger's Invasions.