Well, to play devils advocate one reason Ansel used the framing card is because setting up the 8x10 view camera and looking through the ground glass took too much time.
I do agree that some sort of abstraction into a 'frame' is useful, but not necessarily for the seemingly obvious reason of showing how the composition will be framed. More importantly, it helps us to see the scene in 2-dimensions instead of 3D, and will give us a better idea of what the actual photo will look like. The frame takes what you're shooting out of the context of its 3-dimensional surroundings.
But it doesn't have to be a framing card. It could just as easily be a director's viewfinder, an SLR viewfinder, live-view on an LCD, heck some people can get by closing one eye and using their hands to frame (we've all seen that cliche of movie directors doing this).