It's useful for keeping things 'asleep'.

One glaring example is the avatars on new dynamis summoner NMs. Once sleep wears they'll immediately astral flow, but chuck break on before it wakes and you can safely resleep without needing stuff like spellcast to time your spells accurately. Edit: As long as it isn't Garuda or Titan.

It also stacks with Manifestation so is very useful for keeping a large group of mobs 'asleep' safely without them beating you to death the second they wake up.