what is "population" count info good for?

[General Tacticus]

Population does affect taxes and supplies. Probably ressources as well, but I am not sure...

I like to play the World War scenario (not that I've ever finished it, but that's another problem), and I can tell you that conquering Etruria (north of Italy, belongs to Ermor) on turn 3 or on turn 15 makes a LOT of difference : in the first case it is a very good province; but after 15 turns under Ermor Ashen or Soul Gate, there is nothing left worth bothering : a few scattered peasants, perhaps 2 or 3 gold a turn, and no supplies worth mentioning.

As you see, it's the pop that made the province's worth. I'd say that, special site aside, both gold and supplies are proportional to the population for a given province type.