The power supply transformer has so called Eddy idling currents, even when no power is pulled from it. (heat) The circuit board will always pull SOME idling current from the power supply when the unit is on. (more heat)
The debate whether to keep electronics always on or shut down during off times has gone on for many years. I remember an engineer friend of mine always told me to shut off test equipment to keep the hours off it.
But there is something to be said about keeping a piece of gear at a stable temerature. An extreme example would be a computer that is kept out in the cold and turned on and off for use. The cold contracts everything and the heat causes expansion and eventually can stress the circuit board tracks to the point of causing component intermittents.
That said, I personally believe because the useful life of a CPAP is only 5 years or so before technology makes it "obsolete" or other crap outs rear their heads, (bad blower motor) it makes no difference at all in longevity. Keeping it on all the time will possibly make it perform more stable over its short lifetime due to constant temperature and not having the "thermal shock" and component drift of warmup each time.
I say leave it on, keep it clear so air can circulate and make sure it gets warm, not hot. Always use a fused outlet strip in case something does go south inside it since you will not be watching it 24/7.