Basically, only a hero whose weapon type is "Neutral Staff" can play a role of healer that refill HP of an ally.
Staff
Will never attack
Healers will not be at the front since they receive Atk -50% when they attack. Additionally, they have lower Def so can easily beaten.
How to use
1. In Arena Duels
Healers will not be useful in Arena Duels since the most important thing to win a battle is defeat all foes before your allies receive significant damage.
2. In Hero Battles
One of the victory conditions of Hero Battles is win a battle with 4 survivor. To achieve that, healer's HP regenerating skills are very helpful.
3. In level farming
Healers works very well in level farming. When you want to farm a hero with lower stats, a healer will help the one survive to keep having gained EXP by HP regenerating skills.