Meteion tells Emet/Hythlodeus/Venat of their own personal nightmares bringing down three worlds.
A world where the people thought themselves the strongest, being conquered and defeated (Emet).
A world where the people tried to achieve infinite time/life but were constrained by the laws of physics and gave up (Hythlodeus).
A world where the people cured all suffering but were left with nothing to live for so wanted to die (Venat).
So when Venat is trying to reason with the sacrifices, she realises that they are asking Zodiark to cure their suffering and that her civilisation is going to go down the road of it's own doom.
The sundering makes people have 1/14th of their original power, no longer being powerful enough to prevent suffering and solve all of their problems with their magics. Therefore having a reason to strive to be better and have a will to live.
This answers the problem of evil from philosophy, why would an all powerful benevolent God allow suffering and evil to exist? Because it's nessesary for life and good to exist, proven by Meteion's report.
It's all in the song:
https://genius.com/Nobuo-uematsu-answers-lyrics
It also causes Zodiark to have 1/14th of it's power so that Hydaelyn can imprison it, as Zodiark was summoned by half the population then fed another half again. In Shadowbringers we learn that only a small group summon Hydaelyn... maybe twelve?
The final reason is that in Elpis, only you, a weaker sundered soul with less Aether, is able to telepathically communicate with Meteion who weilds Dynamis.