Motion tracking is used...to track the motion in a scene. Say you have a hand-held shot of a billboard and you want to change the what is on that billboard, you would motion track the shot, which generates information about position, rotation and scale, now you can use that information to take a different picture and place it on the billboard.
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.