The spell restores mana based on your maximum mana (thus making a large manapool interesting but not too important) and grants you stacks of the buff depending on your level. The "buff" got no further benefits but being a second resource to very powerfull spells you get.

You want to use aetherflow on cooldown to make use of these spells 98% of the time. If its off cooldown and you got stacks left you wasted a valuable resource. The exception is as burst damage or burst heal is needed. You might want to save them then to have 6 Stacks available.