All the high precision optical systems I’ve used at work use optical “homing”. They measure a fiducial (in this case, the Glowforge logo is the fiducial) and sets that as its datum point.
If you think of the Glowforge as a traditional CNC tool, limit switches are ideal. They set your x zero and y zero very easily and at the corner of your bed.
If you think of it as an optically driven system, then homing the head with the camera makes sense. You home the head to the camera and then use the camera to define your cuts. Everything is in the same system and reduces some of the tolerance stack. Not to mention the fact that the optical alignment can take out some of the tolerance slop from the magnetically attached head.