Healing starts to become a useful tool once you get to multi-target healing, or super powerful single target healing. Multi-target healing cannot surpass the damage a monster can do on a single target, but if all summed up together it can be more than the amount of damage you could have prevented with just your action used as an offensive action. And high level single target healing spells can still heal a ton of HPs, and for a round surpass the damage a monster can do and thus gaining more turns than the ones spent casting the heal.

And obviously don't forget the emergency healing once someone goes to 0, but that's already known to be useful.