Healing Boost is an offensive attribute, and affects the potency of a character’s healing spells. Characters with a higher Healing Boost attribute will heal more with their healing spells and effects.
Chanters and Clerics are the two classes who benefit most from Healing Boost; other classes receive little to no benefit from the Healing Boost attribute.
A high Healing Boost attribute is important for classes that use healing spells, Clerics in particular. Clerics tend to be the main healers for Groups, Alliances, and Legions, and thus the effectiveness of their healing spells is imperative. Chanters also receive some benefit from Healing Boost, though Chanters often spend less time healing than Clerics.