Why does Marker Tracking ask for height of the image marker?

Mostly asking out of curiosity, but I was wondering why the Marker Tracking asks you to indicate the size of the marker in real-world measurements (centimeters). Doesn't it just scale to the size that the marker appears anyways?

In other words, as you move the marker closer to the camera, your object will scale up with it, maintaining some pixel-to-pixel ratio, so why does it need to know the height of the marker in real-world terms (centimeters)?

Thank you!