Motion tracking can be used to put 3D objects into a scene that were not there during filming or to hold an animated picture in a frame as a camera moves.
Here is an rough demo of how motion tracking can have a big affect. The motion tracking worked great in this.
Another video of a picture being placed in a frame as it is animated.
An PreVis example of a proposed road improvement I did for a developer. This is motion tracked into Google Earth Pro.