There are some libraries, which are available in their versions written in many different programming languages, like for example Lucene, which is written in Java (as they say, 100% pure Java), but has also its versions in C++, C, Perl, Ruby, Lisp and some other languages. And I'm talking about implementations in these languages, not just FFI interfaces.
Why do people do that? I can see one obvious reason: deployment and distribution (and probably development as well) easier when a project has fewer dependencies. But is there anything else? In what situations is it worth it?