If all it provided was an API server then it wouldn't really matter. But this is pitched one level down ie: you can integrate this into your existing app as a library and expose it. I don't know what the utility of that is, but I'm kind of assuming it might mean you can add in interceptors and business logic to customise the API behavior which I think is pretty useful.

So the fact that it lets you do this with any JVM language is actually a key property that gives it very wide application.

Because then people know it can be used by languages that run on the JVM...? E.g. Java, Clojure, Scala, Groovy...

It seems to have been written in Clojure.