As a temp building at the start of each chapter to up the storage so you can maximize spells and then delete it at the end of the chapter. Rinse and repeat
Don't quite see what purpose this AW serves that late in the chapters - unless they plan for many-many chapters to come, and relatively soon. BTG is at least useful for quite a few chapters, even though one can argue that the impact is relatively small when you have PP available. But AW specific to guest races at the end game? With months in between chapters? Anyone hitting the end of chapter 15 any time soon will likely be able to accumulate a ton of PPs if they want to, and once you hit 1000%+ in PPs additional usage efficiency does not matter. We don't have any other use of PPs. Even if you're not overflowing with PPs by the time chapters 16+ are online, this AW is only useful for chapters 16+. Which won't be a meaningful number for a long, long time - unless release patterns dramatically change.
And in-between waiting for chapters? Entirely useless, just like a few other AWs.
And the other one? 20% pop bonus
at L30? So basically if I am running with 20 residences, it will save me 4 residences at the cost of 3x7 and the
cost of upgrading this thing to L30? Nah, I'll pass. Probably better off RR-upgrading my Venar's Rocks III from Halflings if nothing better won't come along in the events (at it's hard to see that there will be no event buildings that are 20% better than residences).
Given the latest event setup, I can totally see some number/level restrictions imposed on residences though. That's the only reason why I haven't embraced "zero residences" approach.