Quoting is a rhetorical device, that is, quoting allusions. Allusions are common in poetry. Allusions can enrich and implicitly express related contents and ideas by quoting stories or words in ancient books.
Allusions are ancient stories (which can be true or fictional) or previous chapters and sentences. Because a story or an article is very rich in content, it is possible to express rich thoughts and feelings with few words and all the meanings of that story or article. Therefore, in ancient poetry, allusions abound.