WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
When grounded (as it always should be) it reduces capacitive coupling between tags/sockets. Most important for RF; less so for audio. Some RF circuits may use it as a local star point, as RF often uses a star per stage.