Manifold is a smarty-pants way to say "varied," "many," or "multiple." There are many good reasons to expand your vocabulary, so you could say the benefits of learning new words are manifold.
Manifold sounds like "many fold," which is what it is — something with many features, like a wallet with lots of folds so stuffed with junk that makes you sit funny when it's in your back pocket. As an adjective, manifold loves to appear in books, like Mrs. Gryce in Edith Wharton's "The House of Mirth," whose "domestic duties were manifold." As a noun, a manifold is a pipe branching into many openings, often found in car engines.