If the first chapter is important, then the first paragraph is the peak of its importance. With only a couple of sentences, the author can filter readers by inducing them to decide whether the book is a good fit for them or not. It is the reason why the list featuring the books with the craziest first chapters came to exist.
P.S.
For any potential librarians, please don't merge the list featuring the books with the first paragraphs and the list featuring the book with the first chapters. While the difference is subtle it is still very present(you of course understand it but I wrote it anyway).