Why demagnitize an optical reader?

Interesting. So there is something going on with the disc magnetically? I don't pretend to have any depth of knowledge or understanding in this area, I just know that it works ;)

It has also been asserted that the inks used the non-read side of a digital disc have magnetic properties that can have a negative effect on the sound of a played disc.

I’m not a big fan of the magnetic ink theory for the simple reason the ink on the CD label could only contain trace ferrous material, at best. I.e., not enough ferrous material to produce a magnetic field of sufficient strength to affect either the CD laser or any electronic elements in proximity to the CD. And certainly nowhere near the magnetic field strength of big honking transformers such as found in a modded Oppo CD player, for example. And perhaps not even near the strength of induced magnetic fields in wire. For the same sort of reason I’m not a fan of the magnetizable metal layer theory, either, as the CD metal layers are quite pure aluminum, gold or alloys. So, there must be a different theory.