If we have managed to build an unambiguous 3D model of the viewed scene, is the process then complete? The answer to this question must be no. Computer vision systems, per se, are only part of a more embracing system which is simultaneously concerned with making sense of the environment and interacting with the environment. Without action, perception is futile, without perception, action is futile. Both are complementary, but strongly related activities and any intelligent action in which the system engages in the environment, i.e., anything it does, it does with an understanding of its action, and it gains this quite often by on going visual perception. Computer vision, then, is not an end in itself, that is, while the task of constructing an unambiguous explicit 3D representation of the world is a large part of its function, there is more to vision than just the explication of structural organisation. In essence, computer vision systems, or image understanding systems, are as concerned with cause and effect with purpose, with action and reaction as they are with structural organisation.
This is a rather remarkable excerpt from "Advanced Image Understanding and Autonomous Systems," by David Vernon from Department of Computer Science in Trinity College Dublin, Ireland.