The sensor-plane mark matters whenever you need precise distance measurements from the image plane, not just an approximate camera-to-subject distance.

Common practical uses include:

• macro and scientific photography, where exact subject distance helps calculate magnification or measure object sizes from photos
• astrophotography/telescope setups, where the camera sensor must sit at the telescope’s focal plane for proper focus
• visual effects and match moving, to verify real camera geometry and distances
• multi-camera rigs and stitched imaging, where accurate camera placement helps alignment and repeatability
• panorama or technical setups where exact geometry is being checked

For everyday photography, most people never need the mark because autofocus and normal focusing make it irrelevant. But in technical work—especially when adapting cameras to optical systems or doing measurement-based imaging—it’s very useful because all lens and optical distance specifications are referenced to the sensor plane, not the front of the camera body or lens.