Having 2 separate windows requires an extra mouseclick for any interaction on most OSes. For example, try to highlight text on a non-focused window.

Obviously a tiny extra effort, and not sure it justifies this feature, but it can add up for some things.

Fwiw macOS does not require an extra mouse click for this, outside of limited circumstances (likely very limited these days) such as X11 apps.

Actually wait... yes it does need an extra click for text selection! Scrolling works without focusing the window though.

Because all the editor keyboard shortcuts and such work in the browser tab