As an instructor, there are many times where you just want to mute the radios, or quickly shut off what's happening on the frequency. Most of the time with older radios your only option is to use the volume knob, or the switching panel to switch off the audio coming from the radios.
I've been using a new trick on some of the new Garmin switching panels, which is to press the COM3 button. Since most aircraft that I fly and don't have a COM3, the button is not wired to anything and it is a quick way to shut off all communications and all radios without messing with the volume knobs or the switching panel.