What was the reasoning behind moving to two mills?

I don't really remember well but it was probably because of patch that changed the collect rates of ressources and the market. When Cossacks 3 was first out people collected a lot of stone because it was the fastest ressource to get, then they would trade stone for food, gold or any other ressource they needed. Players used to do reverse trading, basically they first made the value of gold drop by exchanging it for stone in low amount several time, then they traded all their stone in one go for gold for the best price. You could even play without sending a single peasant to the mill (well you still can but it's not great except on naval map).
Later a patch made reverse trading a lot less interesting. The value of stone also dropped, so players then started to collect wood which had good exchange rate on the market, mill were also more important but one was enough for the first stage of the game.
Another patch made mill collect rate better, food became a good ressource to collect, that's probably why between the two season clash there was a change in the build order and players made two mills and allow more peasant to collect food.

A better player could explain why exactly. I don't play a lot ranked games so I may have missed some important things.