I know it's used for true transparancy, but what else is it good for?
A compositing window manager is a type of window manager. A window manager is software that draws a graphical user interface on a computer display – it positions windows, draws additional elements on windows (such as borders and title bars), and controls how windows interact with each other, and with the rest of the desktop environment.
Compositing window managers may perform additional processing on buffered windows, applying 2D and 3D animated effects such as alpha compositing, fading, scaling, rotation, duplication, bending and contortion, shuffling, blurring, redirecting applications, and translating windows into one of a number of displays and virtual desktops. Computer graphics technology allows for visual effects to be rendered in real time such as drop shadows, live previews, and complex animation. Since, technically, the screen is double buffered, it does not flicker during updates.