EDIT: I understand that the ground wire reduces hum. I'm not looking for a qualitative answer. I'm hoping someone can break it down for me Henry Ott or Bill Whitlock style.
Many turntables, such as the ever popular Technics SL-1200 mk2 have a separate ground wire which attaches to the chassis of the mixer or preamp you have your turntable connected to. It's also a popular modification to modify your turntable such that the ground wire internally connects to the shield of the RCA cable (which is connected to the chassis of your mixer/preamp already), eliminating the need for a separate wire.
My questions is, what is the purpose of the ground wire, especially given that it seems redundant given mods such as https://www.youtube.com/watch?v=3leKhyNinNk.
For the purposes of this question, assume use of Allen & Heath Xone 32 mixer.
Below are the phono input stage, and the point in the circuit where circuit ground connects to chassis ground in the mixer.
And here are some pictures of the inside of my Gemini XL 500 II.
The small black wire connects to the tonearm shell, acting as a shield for the four signal wires. It connects in turn to the rest of the grounds in the turntable via the springs, and in turn the the ground wire post on the outside of the turntable.
A visual/schematic explanation would be fantastic.