Why is it significant that Desdemona could not get "The Willow Song" out of her head?

the answer to your question is true

i guess so you did not explain the answer clearly so i can't

answer: the purpose of the "index of forbidden books" was to prevent the contamination of the faith or the corruption of morals of roman catholics according to canon law, through the reading of theologically mistaken or immoral books. the condemnation of published books, thus listed in the index librorum prohibitorum.

explanation: index librorum prohibitorum, (latin: “index of forbidden books”), list of books once forbidden by roman catholic church authority as dangerous to the faith or morals of roman catholics. publication of the list ceased in 1966, and it was relegated to the status of a historic document.

the 20th and final edition appeared in 1948, and the index was formally abolished on 14 june 1966 by pope paul vi. the aim of the list was to protect the faith and morals of the faithful by putting a stop to the reading of theologically, culturally, and politically disruptive books.