It’s additional space around components showing what’s behind it. So you’re seeing more stuff in between windows, making it look less organised imo. The “whitespace” isn’t really white here. It looks like another unnecessary element crammed inbetween two windows that might as well just sit neatly next to one another, making the windows slightly larger. I also like being able to move my mouse to the edge of things (e.g. the taskbar) without ending up in the whitespace, which causes misclicks for me.

Again, my opinion. Not stating absolute truths here.