The purpose of the holes in the wheels is, as stated, to mount the alignment head.
The actual purpose is not the form of attachment itsself but the fact that the head goes through the holes with calibrated pins and reads the face of the wheel hub. The point being: not having to raise the car to do a run-out calibration. Not only does this save time, it also allows the alignment in the "as driven" state. The act of lifting causes the car to come back down slightly different than it was when driven into place.
Bosch Master, ASE Master, L1
33 years MB technician