Very large screens, very small ones, and detail.
Very large is obvious - you want to project a 100” picture on a wall in your theatre room, then 1) I’m jealous, and 2) 8K helps you there.
Very small - when the screen is inches from your eye, such as VR, pixel density, and even subpixel alignment and orientation, start to matter. Even on phones and laptops, where you are much closer to the screen, you need a pretty detailed image on a fairly dense display to not see jaggies.
Detail - screengrabs from 420p video look pretty terrible on today’s retina displays. even 1080p has to be upsized on most tablets and laptops. 8K images start to get close to the point where most displays, at most sizes, can give you images you can zoom in on or crop and maintain detail.
