The focal plane mark shows the exact position of the sensor/film plane, and it’s useful whenever measurements need to be made from the image plane rather than from the front of the camera.

Common uses:

• Checking minimum focus distance: Lens minimum focus distance is measured from the sensor/film plane, not the front of the lens. This matters especially in macro work, where small distance changes are important.
• Precise camera alignment: In copy work, artwork reproduction, and other technical photography, keeping the subject plane parallel to the sensor plane helps reduce perspective and focus issues. The mark tells you where that plane is located.

So while most photographers can ignore it most of the time, it becomes helpful for close-up work, measurement-based setups, and studio/technical shooting where accuracy matters.