Its currently used to slow the enemy (with forest) or speed you up when entering or exiting a town without roads. Previously clearing was used to add to material, like mana is, but they took that out for good reasons. Maybe it comes back as a boost to materials at some point, but for now it can actually help you defend the back side of your town and give you a turn or 2 warning that someone is a comn' knockn'. So dont go destroying the forest around you until you know where the enemy might come from, and where you plan on heading.
