It’s a way to compensate for belt stretch: https://docs.carbide3d.com/shapeoko-faq/how-to-calibrate-the-machine-for-belt-stretch/
But if you get precise enough results for your needs as is, don’t bother. It’s easy to become obsessed with calibration (I would know…) and at the end of the way it’s more efficient to just compensate for dimensional inaccuracy by tweaking the CAM.