Re: I'm Confused...
I have a Sony Xperia XZ Premium with 4K, ~800ppi, HDR screen. Sure, it's overkill for most cases. By default only select (generally the video/gallery) ones run at 4K, the rest stick to 1080p so no noticeable battery impact. Professional photographers love the 4K for pixel peeping on the go. Just select Professional/sRGB mode.
Even at 1080p "effective" resolution, the 800ppi dot pitch means small details, like serifs on text, flourishes on graphics, or detailing in notification bar icons, look painted. The same for e-Reader pages or text heavy web content. It seems to cross the same uncanny valley we experienced when laser printers crossed ~600dpi and print "grain" just disappeared.
tl;dr: Yes, mostly an indulgence, but a very satisfying one :)
Biting the hand that feeds IT
