Because the gain is so high on a cartridge (40db for MM, 60/+ for MC), the metal in a turntable acts like an antenna, gathering EMF and RF from all kinds of sources (even the very equipment it is connected to) so we need to have a "noise drain". I really don't like the term ground as it has electrical connotations, like with power.
Exactly.
It has to do with the application of Thevenin's Theorem with the implementation of what could be called
star grounding.
It (a separate ground wire) is a method to help reduce ground loops and unwanted common mode currents in the audio signal path, phono cartridge to phono stage input.
Remember that a signal does not just stop when it comes to a ground connection.
A much better and more accurate term would be
return connection. For there to be a complete circuit, all current must have a path to return to the source. What is generally called
the ground is actually the
return path to form a complete circuit.
When the shield of one of the phono cables is used as a ground (return path) for the chassis of the turntable, there is the possibility of unwanted electrical noise, RFI/EMI that is picked up be the chassis to be imposed on the audio signal.
Because almost every implementation for the use of a turntable is likely to be different, there are any number of examples of turntables with and with out separate ground wires working okay. Application of Thevenin's Theorem will show why this is so. No smoke, mirrors or black magic involved.
As I have posted before, it is all about where the signal, whether it is the desired signal or unwanted electrical noise, comes from, where it goes, what happens to it along the way and how it gets home. This is a basic word picture of a complete circuit.