Originally Posted by
Prof
Well...if you've got all three regions in a province, you get a total of 11 building slots besides the cities/villages themselves, giving you the option to build a higher variety of buildings and making it easier to balance public order, sanitation, food income and trade within one province.
Also this gives you the option to have multiple different recruitment buildings in one province, which will make it possible for you to recruit a bigger variety of units without having to move from province to province.