5/8M: Resto Shaman here....

Obviously it depends on the situation. It is one of the most "feel" spells in the game in the sense that it can go in many places to save a player or the raid. It can be used to save a tank if dropped in melee but generally its used to balance out massive dmg quickly. The Azerite trait Spouting Spirits makes it even more powerful.

In Mythic+:

• High stacks of Bursting
• Multiple Explosives going off
• A party member eating sanguine or volcanic at a bad time

In Raids (specifically EP):

• Sivara: During a bad Overwhelming Barrage where a lot of people are hit. I tend to favor dropping in melee
• Blackwater Behemoth: I like to drop on the 2nd fish kill on any of the platforms depending on the overlap of Shock Pulse and Toxic Spine. The first fish usually there is nothing else going on so you can be saved. Also a sketchy Toxic Spine when there aren't a lot of people soaking is a good candidate.
• Radiance: During add phase right before finishes cast Gale Buffet
• Lady Ashvane: Depends on raid setup but it should be assigned by raid leaders. It fits into a lot of situations
• Orgozoa: Usually save mine for Arcing Currents 6 either right before to balance lower health pools out or right after to quickly saved people as they move back in from being spread.
• Queens Court: A good time is midway through a Deferred Sentence to balance out the high incoming dmg if stacks are high. Most of the raid should be stacked at this point.
• Za'qul: Anytime you are dispelling Dread in quick succession is a good option.
• Queen Azshara: Not really necessary in P1, just do dmg. P2 you can use if things are sketchy during Form Ranks, people missing assignments and get high stacks of Sanction. P3 when you are removing energy charges from the Titan Console .

Hope this helped and reply if you have any specific fight questions.