Why is Plato concerned about the difference between mere belief and true knowledge, particularly concerning the issues of justice?

Plato and Gorgias agree that there can be false beliefs, but that there is no such thing as false knowledge. However through persuasion you can make people believe as if they have knowledge, when in fact they have none. So Plato is concerned that in issues of justice, and in courts of law that people will make the wrong decisions, and come to the wrong beliefs because they have been persuaded, but not through knowledge, which can never be false. Thus if we accept that rhetoric is the art of persuasion and flattery, we understand why Plato/Socrates believe it to be dangerous; it can be used to persuade people in the courtroom to make decisions about which they know nothing.