I just noticed this. If you build farm improvements in desert or northern climates, the terrain changes from Wilderness and Low Fertility to Medium and High Fertility.
This is great for barbarian factions who cannot build paved roads, because higher fertility terrain uses fewer movement points than wilderness.
It's also good for Romans and others. When fighting in desert, Numidians and Carthage get bonuses, but if they build farms the terrain around their cities affects the battle maps, making them temperate climates and better suited to the Roman conquorers.
I thought this was a nice addition to the game. (Although it does look kind of silly having large patches of green in the Saharan and Arabian provinces.)