Heal was a priest healing ability, learned by Holy and Discipline priests at level 28. It covers the niche of a cheap heal that can be cast virtually forever. The downside is that it is slow to cast and not very powerful. For a far more powerful but less efficient healing spell, use [Greater Heal]. In situations where speed takes priority over mana efficiency, the quick but expensive [Flash Heal] may be preferred. While both these spells are undeniably more powerful, Heal is more mana efficient, and is a spell that can be used as a filler when nothing much else is going on.
The positive energies of the Light can be channeled to create a healing energy wave. This technique, developed first by the high elves, and later taught to humans, has remained relatively unchanged since its original discovery.
Heal is designed as a cheap and spammable filler spell, designed to counter light damage without denting your manabar much.
If you pick the [Power Word: Solace] talent, you may get more out of your mana by casting [Greater Heal] and using the downtime regaining mana through Solace, rather than chaincasting Heal.
Discipline priests can use Heal to proc and maintain [Grace] cheaply.
Discipline priests can make Heal an attractive option due to [Strength of Soul] reducing the Weakened Soul debuff on targets. With its low cost, regular Heal casts can allow the priest to use Power Word: Shield more often, saving mana, benefiting from Haste ([Borrowed Time]) and preventing spellcasting from being interrupted.
Holy priests using [Chakra: Serenity] can refresh the duration of [Renew] on targets by using Heal (or any other direct heal). One less commonly used strategy for very cheap AoE healing is to cast [Renew] on up to 5 targets and rotate Heal on each of these to refresh the duration of the HoT. This strategy doesn't allow much room for mistakes, but it can be used to keep a small group of people alive virtually forever as long as the damage isn't too heavy.
Combined with the [Holy Word: Serenity] healing spell and [Chakra: Serenity], this spell is often enough to outheal tank damage in leveling dungeons and some heroics, meaning less dependency on the more expensive healing spells.