Mains or machine on/off switching?

[RogerP]

In the instructions of my new compressor is a warning NOT to switch on an off by using the mains input switch but to only use the on/off switch on the machine otherwise the guarantee is invalidated.

Not really a problem but I've been wondering why?

 

[powertools]

When you switch off at the machine it also releases the pressure from the cylinder and that reduces the load on the motor at the next start up.

 

[Chrispy]

Two reasons I can think of, the switch on a 13a plug won't last very long switching a large load like a compressor and the other is to let the auto switch do what it does, as when it comes up to pressure and turns the compressor off it will also releave the cylinder head pressure ready for when it restarts.
Beat me to it!