The art of restoration magic is closely linked to divine spells granted by deities. While not every restoration spell has to be granted by the divines, most of them in practice are. The reasons for this are complex but understandable once some context has been explained. Divine spells are granted to clerics in exchange for worship, service, and adherence to a certain code of conduct. These spells are in turn requested by various followers of the cleric's religion. Most such requests are for healing or curing, so over time, more and more divine spells became affiliated with restoration magic.
Perhaps the simplest form of magic to understand,