The day was created, according to ayearofholidays.wordpress.com/tag/paperback-book-day, to celebrate the first day that Penguin Books began printing paperback versions of their books, July 30, 1935.
It's just another way to make reading books special to new and not so new readers. Where would we be without paperback books. They work for reading on the beach, they fit in carry-on luggage when flying and they can take getting read over and over again.
And, admit it, you needed a reason to celebrate something as the middle summer month comes to an end.