Live Comp (Live Composite) Mode continuously takes a series of images, usually of the same scene. It only adds new, brighter light appearing in subsequent frames to the first image. The huge advantage is that you can effectively shoot a scene over an enormously long period—up to six hours—and never over-expose the original frame.
Thus, Live Comp can be used for light painting, star and car headlight trails, capturing images of lightning, and so on.