It would make sure people who know what they are voting for and understand the system and basic things needed to vote like knowledge of how an economy works or how certain things affect the economy affect people. This would make sure people who are voting and are ill-informed about the decisions they make would have a lower representation in government, overall allowing for better decisions to be made by those voted into power.
I'm unaware if china directly test their citizen on politics, but there are several instances that illustrate the fallacy of political exams. They test applicants who wish to join the communist party, which may also include showing loyalty to their agenda (which some speculate is one-reason why some use extra-legal action against activists , to better there odds of being accepted ). Another example is they screen out "qualified" candidates for the Hong presidency campaign. It not a true election because those in power pre-selected those that wouldn't oppose their views, essentially defeating the entire purpose of voting in the first place.
In a couple of sentences. Policy change everyday and that a essential part of a democracy. So testing the "knowledge" of voters before they even reach the ballot is only useful in corruption scandals
Requiring tests for voting is unconstitutional and wrong. History has proved this. Testing in order to vote has historically been used to suppress the votes of lower income workers and minorities, remember intelligence tests used against black voters?.
Furthermore, who determines what is an acceptable amount of political intelligence to vote? The government, who is of course inclined to a political bias. If they were given the power to determine the political knowledge required votes, they rig this test to only allow voters who lean right or left depending on their party.
Finally, the constitution guarantees that ALL American citizens over the age of 18 who are not felons are granted the right to vote. Nothing in the constitution states that one is required to have a certain knowledge of politics to vote, and for good reason too. Whether you are a political science major or someone who has never studied a day of economics, you have the right to decide who governs you. To deny someone these right's, for any reason, would lead to voter suppression and a failed democracy.