IMHO, it is all about learning a new programming paradigm. If you know Java and then C#, there's not much gain, once both of them have almost the same "type of programming".
But if you get to learn a functional language or dynamic, for instance, you're forced to think another way, and that will probably help you to program better in your favorite language.
It is something like: "It is so easy doing this in {different language you learned}. There must be a better way to do this in {language you already know}". And then you rethink, and build up a more elegant way to do this in {language you already know}.
