Originally Posted by
Fenral
This theory is going to leave Final Fantasy behind (well, except for much-beloved FFIX), and move off into a Tales of Symphonia-ward direction, but...
Aether levels throughout Eorzea are dropping, and each time a Primal is summoned and then slain, Zodiark grows in strength...
Any and all "disturbances" in the flow of aether, particularly when we dissipate some massive entity like a Primal, could be causing that aether to flow from Hydaelyn to Zodiark. The Rejoining has nothing to do with the lands themselves, and everything to do with the balance of aether between the "light" and the "dark." In a way it's like the cycle of souls between Gaia and Terra in FFIX, with Eorzea itself serving as the Iifa Tree. Silvertear Falls is the link controlling the flow between the two worlds.
What they get out of it? No idea, unless their world was on the decline until they started meddling in our world. They may even be trying to correct the balance enough that another way can be found to moderate the flow.