well, most users generally have a small set of operations they want readily accessible with a pointing device at all times, or status information they want to be available at a glance, without obscuring or interfering with window layout.
a persistent window, aka panel, is the conventional way to provide that.
it's simply a conceptual extension of the 'prompt' into a gui environment. nothing has surpassed the panel concept in the past forty years that it has been dominant.
i expect the 'panel' is the primary feature vehicle of any desktop environment - they are nearly synonymous. nearly every window manager also contains support for panels, even if they are not part of a desktop environment that specifically provides them. what do you use?
