Pretty much anything in engine having to do with billions of tris to be rendered with 8k textures.
Generally movie production and AAA game design require a 3090 sized vram buffer to work.
As a bonus. Anything involving landscape creation on the world composition system hits a Vram max rather quickly (unless you close the mini map and work without it).
The real “limit” is always RAM.
The editor likes to keep things in ram for access (like level/tiles/actors/distance fields, etc.
(To the point I’m considering checking if exceeding 64gb works and if doing so allows me to load all 140 landacape tiles at once…)
While working IN Editor (so unimportant for published projects) having enough overhead to work so that you don’t cause the engine to crash is rather critical.
Other than this, there are neatly no benefits in difference between a 3080 and a 3090.
The extra Vram makes stuff like movie rendering a lot easier.
Particularly when trying to render at or above 8k using 8K textures all around.
For everything else, measure the cards performance based on benchmarks.
The better card is not the one with higher vram, but the one which can be overclocked the hardest.
(So water cooled hybrid is the way to go).