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I know that most pedals use around 100k for the volume pot, but ive seen as low as 10k and as high as 1M. what does changing this value do?

teleK wrote:I know that most pedals use around 100k for the volume pot, but ive seen as low as 10k and as high as 1M. what does changing this value do?

This looks like a simple question, but, in actuality, it is not. "Volume" is a function of circuit design and how it is achieved.
If you're speaking of the typical case of signal attenuation going to the output jack, then the answer is a bit simpler.

Quite basically you have the output impedence of the guitar going to the input impedence of the amplifier. Simply put,
if you add anything between the guitar output and the amp input (such as a stopbox), the input of this device should
emulate the input of an amp so the guitar sees what it expects to see, and the output of the device should emulate the
guitar output so the amp sees what it expects to see. If done correctly, you can have a succession of such devices without
excessive signal detriment (tone sucking).

As an example: Look at an old Fender amp schematic.....you'll see a 1M resistor across the input jack. This one Meg resistance
is what the guitar expects to see. That's why you see a lot of modern stompbox designs "improve" on old circuits by adding
a 1M 'pull down' resistor on the input of the circuit. Guitar outputs are more variable. But, depending on the knob placements,
are, typically, low to a few hundred thousoud ohms. That's why you see 100K used as a compromise.