Why is Plato concerned about the difference between mere belief and true knowledge?

Plato is concerned that people who only hold mere beliefs about subjects, while being skilled in rhetoric, will persuade the ignorant public in a negative way.  What I mean is that the rhetor, who doesn’t have true knowledge, operates based on mere belief that some topic is fact.  Since they possess the skill to speak well and believe something to be true when it is not, they will be convincing the audience of something that isn’t true.  This concerns Plato because rhetors who aren’t “just” will use their power of persuasion for personal motives and lead society to ruin.  Plato believes that the “just” will have true knowledge and use this knowledge in a positive way.  For example, if rhetors had true knowledge, the ability to speak well, and a “just” soul, then this would benefit society greatly and many people could learn from this teacher.