Yep, slowpoke is spot on. Ideally you want to plan to use Dissipate. There are times when you really don't need Embrace or fairy cooldowns for the 30s, maybe you used up fairy CDs and Dissipate is your next planned option, maybe you want to plan a fairy switch.

I usually Dissipate during the Seph Ex add phase to sac Selene. Before the last phase is a good option too since there is no healing involved for awhile and can get a stronger spread for ultimate. The adds phase for me was mainly to get full stacks after blowing whatever is left for ED/DPS while Aetherflow is on cooldown, but also get stronger shields for up to two sets of add burn damage, then re-summon Eos for ultimate and last phase. Both are planned uses, allow a seamless fairy switch, and have strong benefits.