Another, closer view, different angle:
The rock adds dimension to the soundstage, tightens the bass, makes the miss smooth like a hot knife through melted butter, and the hits as if they were the wings of angels.
But, in truth,... its a rock from the garden that had a flat bottom, and flat top, was the perfect right height for,.... a hand-steady for putting the tonearm down onto the record.
The arm is a unipivot, which are notoriously tippy, and the rock allows for precise smooth placement of the arm onto the record.