A common use case for Line vs Plane is to find the world coordinate of the user's finger:

The finger touches a 2D screen, but there's a whole bunch 3D points along a line *into* the scene, which are all at that 2D coordinate. So the above script first uses [[Screen To World]] to get two points on that line, and then Line vs Plane to pick out the *one* position that lies on the floor plane (point 0,0,0 and normal 0,1,0). As a result, the Wood block will move to the finger!