Another reason to not keep it on 24/7 is for savings. The power meter probably consumes a negligible amount but even at 1 watt per day, that is still money saved looking at the big picture (365 days a year).
thanks for waking me up in fact, the power meter could be melt and require a power consumption so far I never unplug the power meter and not aware even that device need a power.

My power meter doesen't consume electricity from the socket. It has it's own batteries.