Coalescence can be used to push, save a teammate, or hold a position effectively when used properly. It offers 140 healing per second, combined with the healing orb it is 205.
If an enemy Genji uses dragonblade on a teammate without nanoboost, you can immediately cast coalescence and out heal his damage.

Coalescence can also be used aggressively, if you notice the enemy supports use their cooldowns (e.g. Ana’s sleep dart or biotic grenade, Brigitte’s shield bash) then you can fade into them, throw an orb, then cast coalescence. Along with fire from your team you will deal high amounts of damage with this combo, and should easily kill both supports.

Coalescence should not be needed if your team is hit with Reinhardt’s earthshatter, but are not taking damage from any other sources. You can outheal this damage with Biotic Grasp and a Healing Orb.

If your team is stuck in Zarya’s Gravtion Surge, coalescence is an effective tool to prevent them from dying. Make sure you throw a healing orb into them before casting coalescence, especially if it’s a combo ultimate such as gravblade or gravdragon. Here’s an example of blocking a gravblade combo with coalescence.

You can actually outheal nanoblade and gravdragon with coalescence, but only if you have a friendly Ana who can hit the target with a Biotic Grenade. The healing orb will be required for gravdragon, but for nanoblade you can just use coalescence and biotic grenade.