The biggest gain of Cucumber I have seen is executable specifications. That is, every development task is described somewhere, whether it's pages and pages of a Google Doc or a couple lines in a Github issue. Code then gets implemented, and is almost certainly iterated over time. The question is, does the specification ever get updated to match the code's behavior?

With Cucumber, you have a relatively clear specification (written in Gherkin's Given-When-Then syntax), and it always remains up-to-date because the developer needs to update it as corner cases or new scenarios arise. (Of course, this assumes you are running continuous integration with something like CircleCI and tracking code coverage with something like Coveralls.io.)

I don't think there's anything magic about Cucumber, vs. Turnip <https://github.com/jnicklas/turnip>, or Spinach <https://github.com/codegram/spinach>, or even Steak <https://github.com/cavalle/steak>. But I do think the Gherkin syntax encourages use of clear descriptions that make it easier for a new developer (or yourself, 6 months later) to understand what a feature is supposed to do, and in particular, to avoid accidentally breaking it as you make other changes.