"Your Healing Wave also heals you for 10% of the healing effect when you heal someone else."
Healing Wave is the biggest heal in the shaman's arsenal, and also the most mana efficient single target heal.
The large amount healed and the relatively slow casting time means that it is often useful to begin casting Healing Wave on a target that is taking damage before the target needs the full amount of healing done as the target will continue to take damage while you cast the spell. It can even be situationally useful to begin casting the spell on a target that has not yet taken any damage if you expect them to take a lot of damage during the next few seconds. If the heal turns out to be unnecessary, you can simply stopcast the spell before it finishes.
/Patch 4.0.1 (12-Oct-2010): Level requirement is 7, and the spell is no longer free, available after character creation.