Bumping an older post from this thread, but I think it may help shed new light on the subject now that Q is out and everything.
Reichu wrote:Kaji calls the Key a "backup". Of
what, they don't say, but that might imply that Seele doesn't require the Key for whatever they're doing (perhaps similarly to how they didn't require the Adam embryo in NGE).
Gendo claims the Key can open the doors to Human Instrumentality and has been holding it close to his person since acquiring it, yet Kaji claims that it had been previously kept as a backup. A backup, or in other words a Plan B for Seele.
When Kaji smuggled the key out of Bethany Base, he used Mari's fight with the 3rd Angel as a distraction. Was this done to make Seele believe the Key had been destroyed during the fight, or was the fight simply an exploited opportunity to move the Key while Seele wasn't looking? If it had been presumed destroyed, I feel it would be easier for Gendo to keep quiet about having it for all that time during the skip.
We know that Seele's goal was also the purification of all souls via Instrumentality, and they believed they had achieved that goal when their souls were released by Gendo and Fuyu during Fourth Impact. However, the Impact was stopped at the last minute and Seele's plans never got to be fully realized (and now they're dead, hehe).
So with Seele out of the picture, Gendo now gets to pursue out his version of Human Instrumentality using the Key of Nebuchadnezzar. How is it important though? Perhaps it'll be a suitable replacement now that there are no more Angels? Or maybe now that Lilith's body is gone it'll serve as that end of the deal? Or maybe the Key is simply the ignition to whatever cool ride Gendo and Fuyu are in at the end of the film.
Either way, I think it was important for Gendo to make sure that Seele exhausted their resources for Instrumentality during Fourth Impact in order to betray them once more at the last minute, and since he has the Key in his possession (Seele's old backup) he can now start Instrumentality under his terms.