Maybe you want to detect a man-sized change -- something that affects 50% of the pixels in an area of interest, but you don't want to respond to a mouse running across the floor.
Setting the sensitivity and percentage settings on cameras in your data center or office space can be time-consuming. After all, you want to know when something or someone is moving through your space, but you probably don't want 300 images every time someone walks through the door. Similarly, you probably don't want to be looking through shots of your office space every time a light goes on or off or a fan blows the drapes. Generating too many images can be as bad as generating too few because people will stop looking at them. Setting up your cameras to detect what you want to know about and nothing else is nearly impossible, but you can probably strike a balance that has you looking through mostly at things that matter and only half or a quarter as many "false positives".