Why is this the answer?

The answer is bookworm because this ancient riddle tells a curious story about a tiny thief who eats a book without learning anything from it. This is a very old riddle that uses beautiful, imaginative words to describe something small and surprising you might find in an old library. Let us look closely at the first set of clues: "a man's song be swallowed by a worm" and "words moth-eaten." This is the most important part of the riddle. It is not talking about a regular earthworm that you find in the garden. Instead, it describes a tiny insect, like a beetle larva or a moth, that likes to chew on the pages, glue, and leather covers of old books. To the person who wrote this riddle a long time ago, it looked exactly like the "worm" was actually eating the words and stories, or the "man's song," right off the page! The words look "moth-eaten" and destroyed after the bug has chewed its way through the pages. Now let us look at what this tiny bug is choosing to eat. The riddle calls it "his blinded sentences" and "his bedside stand-by." What is something that has sentences and is often kept right next to our bed for a nighttime read? A book, of course! It is the very valuable "bedside stand-by" that this little insect, the "robber-guest," is trying to destroy as it secretly munches away in the dark. The cleverest part of the riddle is the last line: "Not one wit the wiser. For the words he had mumbled." The word "wit" means being smart or having knowledge. A person can read a book and become much smarter and "wiser" from the stories and facts. But this tiny creature is just chewing the paper for food. It eats the words and sentences, but it does not get any of the knowledge, or any wiser, from the words it has "mumbled" through. So, the bookworm is the perfect answer because it is actually two things in one word. It is the real, little bug that can damage books, and it is also the fun name we give to a person who loves to read so much that they never want to put their book down. This clever and tricky riddle celebrates books while making a joke about the tiny creature that tries to eat them.