The point is, once you get one language to properly run on a given platform and interface with it, you can basically leverage all existent libraries for that platform (with little to no effort depending on the platform).
Much like having Ruby call down to C libraries, you can have JRuby call down to Java libraries. And you might prefer that, for a number of reasons:
- Java enforces memory safety. So when calling down to Java instead of C, there's less reason for concern
- Whatever tool/library you want to use is only (or best) implemented for Java.
- You want to extend an existing Java based system with a language as Ruby.
- You just like Java.
In short, the Java platform is a huge and rich ecosystem. And being able to leverage it with one of many popular languages instead of being tied to Java is both good for the platform and the communities of other languages.
Your choice of language and platform no longer really depend on each other. That means you won't have to write in a language you don't like to leverage a badly needed platform feature, or that you won't lock yourself out of some platform just by your language choice.
Please note, the above of course applies to .NET, although the Iron-stuff doesn't really seem to have taken off yet as far as I can tell. But given there's F# and Nemerle and the fact that C# also allows for terser programming than Java (with Linq for example) and that despite Mono the .NET eco-system has strong ties to Windows, who knows if it ever will.