Yet another buffer supported by OpenGL is the accumulation buffer [2] which can be used for antialiasing, motion blur, simulating photographic depth of field, and rendering soft shadows from multiple light sources. You do not render directly into the accumulation buffer. Instead, you render a series of images, accumulating each into the accumulation buffer, combining the images. Then the accumulated image can be dumped back into the image buffer for display. The effect is much the same as the one a photographer gets from multiply exposing a piece of film.
Motion blur is one use. Imagine drawing a scene several times with each frame corresponding to a slightly different point in time. By accumulating the frames (with decayed intensity for earlier frames), you can achieve an effect similar to motion blur since still objects are sharp but moving objects are blurred by their accumulation in slightly differing locations.