The primary purpose of the impedance switch is to protect
the AVR's amplifier output stage from destruction...
Todays AVRs are typically built under very tight component material cost budgets, so the brand strives to save $..
If one is running low impedence/sensitivity full-range loudspeakers and being driven to higher SPL levels, it is easy for the AVR's power supply and amplifier output stage to be stressed. And amplifier stage failure is one of the major reasons AVRs are sent in for warranty repair...
Its basic function is too limit the current
available, by reducing the power supply to a calculated max
voltage output. Also when set to a lower impedence position such as 4 or 6 Ohms, the dynamic range capability of the amplifier is reduced
as the amplifier stage has max output (voltage & current) without this limitation. So for the highest sonic performance
it is best to leave
it in the 8 Ohms position.
Just my $0.02...