It is a switch that allows tubes to warm up properly before starting to amplify. For a tube to work properly, it needs to have its 'cathode' very hot. If we start to ‘draw’ electrons before cathode is at an optimum temperature, we can damage them by the 'cathode stripping' effect.
Not 100% necessary. It is advisable is that the amplifier does not emit any sound during the first moments of operation in which the tubes are getting hot, since electrons would be torn from cathodes as they were with a hammer. A logical substitute is to lower the amplifier's volume to zero and raise it when the amplifier is hot.
Of course it is not. That's why they keep manufacturing amps with a Standby switch. Even though, the wear of the tubes starting operation with that small background noise is practically negligible.
It is not advisable since, depending on how the power of your amplifier is designed, you may be exposing the filtering capacitors to excessive voltage if the amplifier is on, but the tubes are not consuming power. Some people also say that a 'cathode poisoning' can occur because the tube is not polarized, but it is heated. In this case I am personally more skeptical since, if the phenomenon occurs, it is in a negligible amount.
To turn on your tube amplifier:
To turn off your tube amplifier:
The Standby in your tube amplifier is a tool like any other. Probably playing with all the controls of your amplifier set to 10, you will damage it in a few hours. Few people are asking around 'Why can I put everything to 10 in my amp if it is bad for it?'
Use the Standby only for short periods of time, from thirty seconds to a couple of minutes, to turn on and off your amplifier, and its use will be very positive in the duration of your tubes. Avoid using it for long periods of time with the power switch already connected, since you can cause serious problems in the power section of your amplifier.