Healing sources are important for slower battlegroups, to help deal with constant slow damage, such as those from long range champions or one on one fights.
Common self healing sources are Regeneration, Blessed, Heal Self, Assimilate, and Eater abilities. Heal Champion is common for healing a specific champion, and Heal Mass, Healing Breath, and Divine Favor.
Note that one weakness of healing abilities is that champions which have already been dstroyed can't be healed. Thus Focus fire is one way to deal with healing sources.