I’ve seen 5K some.

IMO the only ostensible benefit is for computer type stuff. It gives them more headroom to upscale content well, to avoid anti aliasing or blurry, scaled UI rendering, stuff like that. 4:1 rendering (to save power) would be quite viable too.

Another example would be editing workflows, for 1:1 pixel mapping for content while leaving plenty of room for the UI.

But for native content? Like movies?

Pointless, unless you are ridiculously close to a huge display, even if your vision is 20/20.