[adamwk]

What language only allows concatenating a non-empty array?

And it still doesn't explain why string concatenation being a monoid is useful in ruby. It's useful in Haskell because implementing one of their category-theory typeclasses means that type now inherits a stdlib-sized API that works for any monoid, not concrete types. But even Haskell hasn't proven that it's worth the jargon or abstraction after all these years; every other language remains productive without designing APIs at such a high level of abstraction.