I know it changes the number of votes you have in the united council, and it increases the chances of getting your prefered government and being able to go to war if you're using another government that imperial.
But reading some strategies for gigantic maps, most people say it's imperative to have 100% approval, even in the early stages of the game where you don't have a special type of government. Is there something I'm missign about the effect of a high approval rating?