It gives the render a "path" through which it can render a window or other such hole in some geometry. Makes the engine, I surmise, not have to figure out how to get/read rays through geometry making it work more efficiently.
Nodally, I tend to mix a portal and the material I want to show on the wall. In this way, I get the look and the more efficient effort of the render engine.
i7-4770K, 32gb ram, windows 8.1, GTX Titan and gt 620 for display