Paul is saying here that we bear witness to the benefits which the ruler gives us by paying him a salary. The taxation system may seem to be burdensome and annoying, but Paul turns it into proof that rulers care for their people. Why, after all, do we pay taxes to the emperor? Is it not because he provides for us? We would not have paid it in the first place if we did not know that we are the ones who benefit from this government. It was for this reason that the men of old agreed that rulers should be maintained by the people, because they neglect their own affairs in order to devote themselves entirely to the public welfare, spending all their energy in order to protect us.