Otherwise, why waste its time testing something that it already believes is true?

Because it might be false. If your utility function requires you to collect green cheese, and so you want to make a plan to go to the moon to collect the green cheese, you should know how much you'll have to spend getting to the moon, and what the moon is actually made of. And so it is written, "If you fail to achieve a correct answer, it is futile to protest that you acted with propriety."