what a healer does to my knowledge:
when a unit is dealt lethal damage, the game rolls based on your available healer to see if they are wounded or dead. wounded units in your army can recover, wounded enemies can be captured
Healers help wounded units recover to their normal usable status outside of battle. they also help heroes recover missing hp (anyone who isn't a hero just has 2 settings, injured or not)
generally speaking wounded heroes skills don't work, soit's generally good to not get them too involved in combat, so they can help the party recover afterwards.
PS: this doesn't really belong in the "feedback and suggestions" forum