What are some unexpected or unknown benefits from learning Esperanto?

If you immerse yourself in early Esperanto literature (before World War II), you end up learning a lot about Mitteleuropa high culture—and indirectly, a fair bit of German. There’s a lot of Heine, and a lot of emulating of Heine. Esperanto poetry is also a whole lot more formalist than English-language poetry (another Mitteleuropa thing), so you learn early that a Shakespearean sonnet is not a proper sonnet.