The Binding of Triad

[Rocl]

The first answer is somewhat easy to answer: we know from both flavor text in the Sephirot quest and the Binding Coil of Bahamut storyline that the Allagan kept Primals/Eikons imprisoned and materialized in Hydaelyn by inducing a sort of a comatose state on their followers. Basically, they goad the tribes to summon a deiform, capture them by whatever means they use to capture Primals, and then capture the summoning population to sustain the deiform's existence.

The second question I'll have to dig up old threads and go over my Historia regum Allag before making any concrete claims but the immediate thing that comes to mind is that capturing a Primal prevents it from being used against the Allag again. That is to say, the primary purpose of capturing Primals wasn't for their unlimited energy or whatever, but to prevent their enemies from continually resummoning it.

A lot of the nitty-gritty of summoning is still unknown. We know the basic process: you offer aether in the form of crystals, and your fervent prayer attracts a soul from the Lifestream. The aether from the crystals is absorbed by the soul and shaped into the god to answer your prayers. This might answer our questions. Capturing the soul which answers the prayer prevents the resummoning. The aether used to summon it? A huge bonus. The Triad and Bahamut were summoned in a much more aether-rich era, their power wasn't so much the devotion of their followers or the soul which answered the call but the overabundance of aether available on Hydaelyn at the time. And so the Allag slowly drained them of their aether while imprisoned and ensured they wouldn't be summoned again--the deiform's soul trapped in the material plane, unable to dissipate into the aether.

I've spent like two hours now trying to wrap my head around the aether/worship divide here. We know worship/prayer is how a Primal is formed, but we're less sure of how it affects them once summoned. Lady of the Vortex (MSQ44) seems to indicate it can augment a deiform after the summoning rites, but we don't see this come up much more. I'm not making much headway into it, so I'm inclined to just stick with my gut.

The point to containing primals is to prevent their summoning.