The witness is a useful fiction on the way to something truer. It names the position from which experience is observed without being pulled into it. In Advaita it is called sakshi — that which lights up every object but is never itself an object.
The non-dual traditions eventually turn even this on itself: if the witness is separate from what it witnesses, duality remains. Ultimately the witness and the witnessed collapse into one awareness — or, said differently, it becomes clear there was never more than one.