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.
What I think made screens feel different was as a grouping mechanism, usually driven by a single application (though in more recent versions of AmigaOS you could open "public screens") that would manage the layout for that screen. You can do that with virtual desktops, but it takes some effort to simulate the behaviour.
I'm slowly iterating on something like that for my wm. I now snapshot the (by default tiling) layout and restore it, letting me "open" and "close" screens/virtual desktops, and re-open the applications on them, so instead of opening a single application that opens a screen, I will open a "project" and it will open a desktop and multiple applications on it whose windows will snap into place. I'm not happy with it yet, but it feels somewhat closer to me to how it felt to use Amiga screens.
Some systems like Xsgi went further with having several visuals used for predefined purposes in their modified Motif versions
- 3D apps (games) render at a resolution less than the desktop for performance reasons (even in fullscreen mode to avoid switching the video signal)
- Similarly, streaming sticks/TVs render the UI at 720-1080p and overlay on hardware decoded 4K video (very common in anything that's not a console, Apple TV, or NVIDIA Shield)
- Handling non-high DPI apps on a high-DPI desktop, or handling systems with mixed DPI displays
- Combining HDR and SDR content on the same desktop. Same with deep-color apps.
- UI effects like the OS X genie effect, app thumbnailsI love the screen concept, but the different resolutions and colour depth are the weakest reason for them on modern hardware.
Circa 1985 hardware has significant limitations and it's leaking through abstractions both ways. A modern program supporting deep color writes to an array consisting of R16G16B16A16 values, next to a legacy program writing R8G8B8 and even a window with 10-bit 4:2:0 YUV HDR video. On the Amiga, the hardware leaks through by having to deal with separate bit planes and palettes.
Similarly, the 3D hardware in a modern GPU easily upscales a legacy application's surface 1.25x to handle the mid-DPI laptop screen I'm using right now. 1985 hardware can't do that, because you're fiddling with the DAC output, you can only have one screen/ViewPort scan line, no overlapping regions, and if you look carefully, blank lines between regions for the hardware to reconfigure.
Many applications had settings to run either in windows on the Workbench screen, or on a separate screen. Some would then let you choose between windows on a separate screen or running effectively full-screen. And using screens like a grouping mechanism like that was overall the more common use of screens other than for e.g. paint applications that needed the different colour modes.
It felt like a less offensive version of Windows MDI concept.
It's not that there are no applications today that approximate it because there absolutely are. It's more that on AmigaOS this was ubiquitous and expected and you didn't need to do anything to fullscreen these applications - only small utilities tended to lack options to run on their own screen.
In later versions of AmigaOS, you could also open "public" screens that like the Workbench screen allowed windows from multiple applications on the same screen, somewhat like virtual desktops, but that somehow felt less compelling, I think because you then relied on separately starting multiple applications.
Even on big screens, I like MacOS's two level cmd-` to cycle between windows in the same app and cmd-tab to cycle between apps, and App Expose (4 fingers down on the trackpad). I know what I want to find is in another browser window or spreadsheet window.
I honestly don't know what will feel ergonomic on a modern setup - I'm chasing nostalgia in part, but at the same time I'm fully aware that I remember it with very rose-tinted glasses, so I'm trying to stepwise approach some mix of the nostalgic elements that works with my existing setup.