This type of Hero can restore a teammate's Health either by Active Skills or skills in their Rotation. Healers are quite necessary in about every instance because a higher effective Health will sustain the life of your team. Dedicated Healer Heroes have an Active skill that heals a great amount which in turn fits them into their main role. Typically in most fights, whoever can heal first and the best is the victor, especially in Arena.
NB: Ion will prioritize and heal the lowest health percent teammate in his rotation. Athena heals allies and damages enemies with her AOE "Parthenoseia". Hades can target the lowest health percent hero (yours or your opponent's); heal it if its an ally or do magic damage if it's an enemy. Hecate can nearly heal to full with a hidden ability that activates automatically when she kills a Hero with a skill. Aries heals the weakest ally with a heal over time, eve if it's her summoned ram from her ult.