>Maybe it's the influence of the fullscreen only, unitasking paradigm from phones and tablet computers, but it just doesn't make much sense to me on a desktop computer.

Completely the opposite. It's the influence of having computers powerful enough to have massive multitasking. Arranging windows like papers on a desk is easy when you have a handful of simple programs and a couple of browser windows - it's very different when you have 100 tabs with their own task-swapping logic in front of 20 other programs. You've already lost easy swapping so you might as well gain the extra pixels.