You can't ban books, as I'm sure you all know. You can stop stocking certain books at certain locations, but freedom of the press means every and all book is allowed to exist.
They explore different themes that a lot of other books tend to ignore. Being the first or the most popular book that touches on different themes is enough.
Some books, like Catch-22, The Grapes of Wrath, Flowers for Algernon, The Jungle, The Great Gatsby, Lord of the Flies, Of Mice and Men, Slaughterhouse-Five, To Kill a Mockingbird, The Handmaid's Tale, Fahrenheit 451 (unlisted) and 1984 are on there for having dark themes that the book is based around.
Hemingway's books (A Farewell to Arms, For Whom the Bell Tolls, etc.), Catch-22, The Jungle, and Slaughterhouse-Five explore dehumanization and the horrors of war. To Kill a Mockingbird, A Clockwork Orange, The Great Gatsby, Of Mice and Men, and Flowers for Algernon all are great, dark criticisms on the world and civilization. The Handmaid's Tale, The Grapes of Wrath, Fahrenheit 451, 1984, and The Jungle are all enormous criticisms on the power of the government, the ignorance of the common man, and dark implications of a world to come.
They're just different from the status quo, that's all.
(By the way, anyone who hasn't read the books I listed above really should. But give Hemingway's books and To Kill a Mockingbird a skip, they aren't that good.)
Some books are on there for hilarious reasons of influencing children. Harry Potter, probably for 'promoting witchcraft'. Mark Twain's books, for the use of the word ******. And Tango Makes Three for showing samesex relationships in form of a children's book (protect my babiez's errs!). Are You There God? It's Me Margaret for talking about puberty. Forever for implying that sex is a natural thing that strengthens current and future relationships.
Nothing raunchy at all about the books above, there's simply some people who don't think children should be reading them. To which I say, fuck no, those are books children should be reading!