Some amplifiers I've encountered have what amounts to a gain adjustment that
consists of nothing more than a voltage divider at the input, oftentimes
followed by an op amp buffer, etc. Basically, the gain adjustment is done
prior to the input stage of the amplifier.

Some amps that utilize global NFB have a voltage gain that's defined solely
by the value of one resistor (per channel). Why introduce the added
components associated with the input circuit I described if that resistor
could be replaced with a potentiometer? Is there something that makes that
resistor unsuitable for gain adjustment? Somehow more prone to noise?
Thanks.

PS - I'm not looking to modify or construct an amp in this fashion; I'm
curious more than anything else.