It's a feel thing. I have two screens, with a minimalistic bar taking about 15 pixels on the top side of the secondary monitor. No opened apps or anything displayed there, to see that I'll just press Alt tab or the plasma overview (very gnome-like). That feels less cluttered than a bar that, to have the floating effect, steals me more vertical space than what I have.

This is very personal so it's nice that KDE let's us do whatever we want. IDK about the default choices as long as they let us change it to whatever we like, so I don't really care.

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.