The idea is to be able to visualize how your new HDTV will look in your room, be it Plasma, LED, LCD or even a projector screen, before you actually buy it and mount it on your wall; see how your interior design takes a new addition and what size just feels right in your living room (or whatever room really). It's better to have an idea about these things before you stumble upon a handsomely discounted tv on sale, isn't it? Note however, that what you see is not necessarily what you get; don't rely solely on this preview when buying a TV.