The CDF used to be a part of a larger system of censorship where all Catholic scholars would seek imprimater  and nihil obstat from their local ordinary and his theologians, with the CDF as a resource for that process.  Going outside that process could get you on the Index of Forbidden Books.  The CDF is a vestige of that process, most of which was gotten rid of in Vatican II and the papal degrees thereafter.  Nowadays, if the CDF condemns your book, sales go through the roof.  If you look at books that were once on the Index, you will find some of the best works of science and literature.  So much for the infallibility of the Holy Office.