Those Holy Knights are Incarnations of Imu, They enforce her Tax
Imu similar to Big Mom's Soul Pocus has instead Ability to extract "Desires" from People's Wealth & it's in the Form of a Light
So anyone who joins WG must pay Heavenly Tribute & not realizing that Imu is making Light Energy out of them
If some Nations refuse (Such as those Eight), Imu can use her Ability by Force on everything They own
Cuz similar to Big Mom's Condition being "Fear", Imu's Condition is "Betrayal"
Holy Knights can use same Signature Attack as Imu & the reason their Silhouette had Patterns that were used when Kaku & Kalifa were gonna eat their DFs, is cuz Imu & Holy Knights create Devil Fruits! similar to how Big Mom creates Homies
Lulusia Kingdom wasn't Destroyed, it was "Absorbed", as in turned into Light & Absorbed
In other words, those 16 Light Beams were going up, not down
Why 16? Well cuz Scientifically it's said that there are 16 Basic Desires, which is what Imu's Powers revolve around
Taking all of 16 People's Desires is like Peak Slavery, that's why the Number is referenced a lot with CDs, including the Manipulator Doffy
Imu's Light Ability is reason why there is Room of Flowers, Enies Lobby never goes Dark, All Pacifista & Seraphim have Light Powers, Kizaru serves CDs ... etc
However, People's Desires even though They can be turned into DF Abilities by Imu, They still only work with her Presence (Because They are basically Holograms that only her can turn Tangible), so for DFs to be truly created, they require both Light of Desires (Probably absorbed by Sun Tree Eve) + Imu's Presence (Who can Copy herself)
Meaning each & every DF contains Imu's Presence which is what makes those Abilities that originate from People's Desires able to change Reality.