The jig is for repeatability, Ken. Without the stop, it's difficult to hold the moulding in exactly the same place each time you cut a new piece which throws off your angles. It's also easy to do other corners besides the standard 90 degree using this type of jig. Where Andrew has his stop set at the bottom of the jig, I have mine screwed to the fence and push the moulding up to it(I don't use no stinking clamps on mine). Basically you're removing considerable math using a jig like this because you're only using the swivel on your mitre saw vs the compound angle style...you're actually cutting the trim as if it were already installed on the ceiling/wall.
Half of communication is listening. You can't listen with your mouth.