Heads, how much gain?

Actually both. But starting with cam. I have always, and seen as well, cam degreeing at the centerline mark. (I'm very sure there are reasons to use the advanced/retard marks. Or why have them?) I just never had. My question? What can one expect by advancing? With relation to lets say ignition timing. And/or carb settings? ( I'm guessing you are going to say none. That advancing cam setting is only to move RPM power band earlier?) But I suspect it will affect ignition timing as well?
My point was that to make power with iron heads you need more duration but in order for you to use it you might need to advance the cam to keep the power peek happening before the heads stall thats it.