Icons on the desktop! The minimize button! Visible Panels and docks! Just kidding, don’t have a heart attack. :) Well actually there are some things… just not those! The full list can b…
Basically, you can open some widgets inside a standalone window instead of attaching them to a bar/desktop, making them act like some kind of standalone application instead - including losing all their state as soon as their window is closed.
The only widget I've found in any way useful as a detached window like that has been the sticky note, and even there the usability is limited compared to just opening kwrite - or any other simple text editor.
It's definitely an interesting - if quite useless and potentially confusing - feature, but it makes complete sense to drop it from core and instead let it live as an extension instead, since it's quite literally just a krunner runner anyway.