Why use over powder wad?

[LIMPINGJ]

Kynoch Nitro Express Wads - Size 2 .400-.500 NE x100
Kynoch
BE THE FIRST TO REVIEW THIS PRODUCT
4
IN STOCK
SKU
kyno-118390
Unique case filling system developed and used by Kynoch on all their big game ammunition and now available to the home loader.
https://1967spud.com/media/catalog/p...-3321-2t4j.jpg
Looks like Kynoch Ammo uses something like backer rod to fill the space between powder and bullet base.

[martinibelgian]

The Modern Schuetzen Rifle - Charlie Dell and Wayne Schwartz

Also https://castboolits.gunloads.com/sho...hamber-ringing

I agree with the source, not with your interpretation. And that link mostly is interpretation.
Remember, this is Not new, just forgotten. The french scientist Vieille discovered the phenomenon and its cause late 19th century, we kinda forgot it...

[Huvius]

I use a card wad in my black powder rifle cartridges because in Eley did in the 19th Century and they knew more about the subject than I ever will!

That said, I believe they are to protect the base of the boolit as LedPots photos demonstrate.
An added benefit is when I knock over my cartridges or one of mine which use a slip fit loses it's boolit, all of the powder doesn't spill out!