Why do you render a video? It's quite simple. You want your video project to playback in real time with little stutter and with all effects and everything else in place.
When rendering anything, you may be adding effects, layers, adjustments in color, multiple audio tracks, etc. In your master transport window, you would generally want to see what you would consider as close as what you would call your final product or at least what will be cut and sent off to the next person to continue in the chain.
No one in the right mind would want to edit a video and watch it essentially lag or skip frames.