The heavens protect those who serve. Some better than others. And then sometimes you have to protect yourself.
This can be a useful spell throughout the game, the boost in effective health points it gives your hero can be quite significant. It can keep your spellcasting hero alive in many situations. If you have a choice between using a healing spell or this spell to give your hero a similar number of health points, you might be better off with the healing spell as this can be dispelled.