Originally Posted by Platefire
2-Allows you to mute your amp while you take a break with the heaters and bias still running so you have instant sound when you come off break.
I stopped putting standy switches in my builds after reading Merlin's take on it; which was shortly after I watched a JJ GZ34 do exactly what Merlin says can happen when you hit a rectifier with a sudden HV spike.
I now put a 'mute' switch in the amps. Routes the signal to ground just before the PI.