I occasionally want two browser windows open at the same time, to compare two documents: computer code, music scores, lyrics, ... I can already do that by opening a document in a new window instead of a new tab, and arranging the two windows side by side or one above the other. Side by side windows are best for line by line comparisons but my desktop monitor has a 5:4 aspect ratio, not the more common 16:9, and side by side windows are rather tall and thin.

So I don't think being able to split a tab in the same window will add much. Saves arranging the windows, I suppose.

An Android, fwiw, you can split the display but not have the same app running in both. So I can have, say, Firefox and Chrome in the screen side by side - on a tablet.