An additional benefit to using an AoE heal such as Prayer of Healing, rather than healing multiple targets individually, is the convenience and lack of time lost selecting each target. AoE healing may therefore be preferred even when not the most mana efficient option.
As Prayer of Healing only heals for a small amount, it is useful for avoiding overheals (and so conserving mana) when multiple targets need 'topping up'.
While Prayer of Healing only has a healing radius of 20 yards, it has a cast range of 40 yards.
The area of effect is centered on the target, not on you, so it can heal players up to 60 yards away from you. This allows you to heal players that would otherwise be out of reach.
Line of sight is only checked on the target, not on other players receiving the healing. This is useful in arenas and if a boss locks you out of its room. In such situations, cast it on yourself.
Now heals the target and the 4 nearest allies (was 5 random injured targets).
Patch 7.0.3 (2016-07-19): Now available only to Holy. Healing increased by 198%. Now heals allies up to 20 yards from the target, down from 30 yards.
Hotfix (2013-03-27):For Discipline Priests with Divine Aegis, Prayer of Healing criticals should now heal for the correct amount.
Hotfix (2012-10-12):Now provides 25% more healing.
Patch 5.0.4 (2012-08-28): Now learned at level 46 (was 44). No longer available to Shadow priests.
Patch 4.0.6 (2011-02-08): Effectiveness has been reduced by 15%.
Hotfix (2010-11-23):Prayer of Healing is now properly healing all of the target's party members, unless they are further than 30 yards away.
Patch 4.0.3a (2010-11-23): Prayer of Healing has had its mana cost reduced by nearly 30%, and its base points and Spell Power coefficient increased by 20%.
Patch 4.0.1 (2010-10-12): Now learned at level 44 (was 30).
Patch 3.2.0 (2009-08-04): The percentage of spell power this spell gains in healing (per target) has been reduced from 80.7% to 52.6%.
Patch 3.1.0 (2009-04-14): This spell now heals the target's party, rather than being limited to just your direct party.
Patch 2.3.0 (2007-11-13): Now gains additional benefit from spell damage and healing bonuses.
Patch 0.7 (2004-06-15): Mana cost and heal amount increased.
Patch 0.6 (2004-04-13): Cast time reduced to 3 seconds.