It's useful to learn when DPS limits can help push phases particularly efficiently, or when it's beneficial to group monsters...
Caster in Tam-Tara's final boss to drop the last wave of adds.
Caster in Haukke's final boss to drop the last wave of adds.
Melee in Qarn's Golem when the soulstone is shattered.
Caster in the last leg of Wanderer's Palace after the whole hall is pulled.
Melee in Halatali Hard's second boss to kill a Catobeplas' eye (if your team is undergeared and can't rush it).
Caster in Sastasha Hard's second boss to kill the enormous cluster of adds, or in the last leg to clear the whole hall.
Melee in Qarn Hard's second boss to kill the Sabotender Guardianguy.

These are just dungeon examples of when it's an excellent utility instead of a flashy killshot. Always circumstantial, since overgeared parties are common, but always possible and justified when the time's right.