Along with minimizing screen repainting, overlay planes can be utilized to generate transparency effects efficiently. Usually overlay hardware supports a special transparent pixel value. If this value is drawn into the window, the pixel value in the layer below ``shows through.''
Imagine an application which generates annotated weather maps of the United States. The map of the United States itself is unchanging but the front lines and temperatures and other symbols which are painted on top of the static map do change.
By utilizing overlay planes, we can quickly redraw the weather map by only redrawing the meteorological annotations for the map. To do this, we draw the static map in a window located in the normal planes. Then create an overlay window as a child of the static map's window. The background pixel for this window is the overlay planes' transparent pixel. Effectively, we see through the overlay to the map. Now we can draw the annotations in the overlay window. When a new set of annotations is to be drawn, we clear only the overlay window, leaving the static map untouched, and redraw a new set of annotations.
Of course the same application could be written without overlays but overlays allow a much more efficient implementation by eliminating redrawing of the static map.
Video game style animation can also be efficiently implemented using overlays. Space ships, asteroids, and sneaker-wearing hedgehogs can be drawn into an overlay window while an intricate background window scrolls by in a normal plane window using transparency effects.