People ban books that they believe are unfit for consumption due to themes, plots, ideas that are opposite of what their ideals are. Often time, in modern-day, books for children and teens are challenged and banned because parents, teachers, or other adults feel topics are too mature for their age. There are, of course, other reasons that are a bit more political which are similar to ancient Roman reasons:
In 8 CE, Augustus sent the poet Ovid to exile and removed his unsavory Ars Amatoria (“The Art of Love”) from public libraries. This banning and exile occurred because Augustus’ reign was centered around the morales of the Roman family; while the Ars Amatoria was a bit more risque about love and romance than emphasizing the Roman family. Ovid died in exile in Greece eight years later.