Maybe its me, but i believe the primary or secondary reason for screen is different screens had different resolutions and color depths, both of which dont really exist these days on any major OS.
It feels like the concept faded with the advent of large VRAM and fast fill rate graphics accelerators. I remember old X Window System workstations supporting multiple concurrent windows with different color depths and modes. The hardware would compose windows from memory during display scan-out, all to save VRAM.
Color depths, not modes per se, and it's more that XFree86/X.Org by virtue of being lowest common denominator didn't have optimizations based on it, and over time GTK and others forgot they exist.
Some systems like Xsgi went further with having several visuals used for predefined purposes in their modified Motif versions
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Some systems like Xsgi went further with having several visuals used for predefined purposes in their modified Motif versions