https://www.leagueoflegends.com/en-us/news/game-updates/patch-13-21-notes/#patch-items

These dmg buffs on ally now count as your own dmg:

Ivern: W - Brushmaker

Leona: Passive - Sunlight

Lulu: Passive - Pix, Faerie Companion; E - Help, Pix!

Nami: E - Tidecaller’s Blessing

Renata Glasc: Passive - Leverage

Sona: Q - Hymn of Valor [Empowerment Only]

Yuumi: Q - Prowling Projectile [Empowerment Only]

So you proc spellthiefs and aery when an ally procs your pix dmg or whatever. This makes oblivion orb do much more dmg because in a lot of situations you don't directly dmg an enemy, but you buff someone who does. See https://www.legendsofleague.gg/league/post/177/healing%2520reduction%2520efficiency/ - the healing reduction decision is just about how much dmg it does, which is as much about how much you apply it as how much healing they have.

I also wonder if it makes helia viable:

-on lulu lategame you don't dmg enemies much since you're using E to shield, but maybe now with pix dmg you proc it more often

-nami e not procing it dissappointed me on helia's release, but now it works as you'd hope

-sona might proc it more often off of ppl autoing with her q aura, idk how many more procs that is but it might be good

These champs all like the ap from helia so if the item effect was good, and the change to counting dmg procs as your own might make it good.