> Why would I want a webpage which stops showing additional content after ~1200 pixels wide to take up the entire of my 2560px wide monitor?

Personally, 80% of my web usage these days is hacker news and wikipedia. Neither of which do this.

The rest is probably majority dev docs (crystal atm), and I'm not aware of any dev docs that do this either.

The point here being that not all websites exhibit this behavior.

Because it removes the clutter of your desktop + other windows. I think many would agree. Sure, there are times you need to see windows side-by-side, and there is affordance for that. But mostly, a person is doing one task at a time.

As my displays have gotten larger, I've found I want my windows to take up less and less of them. I may occasionally full-screen something, but it always feels incredibly difficult to deal with. As primarily a Windows user, I've more than once wished I had a "fit to content" button like Mac's.

Just another instance of different users having different patterns.

I feel like the expected pattern to deal with clutter in MacOS is by hiding rather than just covering. This is often forgotten about but you can alt+click on an app to hide all other apps but the one you clicked. This is even in possible in Mac OS 9. In modern macOS you can invert the behaviour by turning on single-app mode through the terminal. It's then alt+click to show multiple apps and just click to hide all but the app.

I'm this person. I have a hard time focusing on one, never mind more than one - in a similar vein, notifications are also disabled / minimized.

To avoid everything else distracting you.