The only difference I can see is that you might have for example four windows 1, 2, 3, 4, all taking half of the screen. On a compositor like Niri, you can scroll so that you can see windows 1 and 2, or 2 and 3, or 3 and 4. On vertically scrolling one, you can see 1 and 2 or 3 and 4 if I understand it correctly. This is much more noticeable if you work with many smaller windows, just like on the screenshots from the article and repo’s readme. I usually use only one or two windows per virtual desktop, so what you suggest would be more practical for me. But I use only notebook, and I can imagine using Niri on some hi-res ultrawide monitor.