This is a great question. By book 10, Plato has now finished his discussion on what is justice and now he addresses the topic of poets. In a surprising way, Plato actually banishes poets from the city. He does admit to some artful beauty of poetry, but he sees that poetry has great dangers. Hence, he is willing to sacrifice poetry. He mentions two points.
First, poets speak as they know things, but they do not know what they are talking about. At best they see shadows and imitations and not the eternal forms, which are only possible to see through philosophy. Hence, they mislead people through false knowledge of truth.
Second, there is a practical effect to all of this. Many people in the city will be affected negatively by the poets. In other words, their appetitive part of the soul (the basest part of a person) will be prodded to control the person. Reason will be debunked. This will cause problems.
Finally, Plato mentions the myth of Er, to speak of the immortality of the soul.