what is the purpose of the function annotation method in the Facebook JavaScript SDK?

what is the purpose of the function annotation method in the Facebook
JavaScript SDK?

I'm starting using facebook javscript SDK, and I found a interesting thing
when I read the source code.
The code example is as belowed:
/**
* Annotates a function with a meta object
*/
function annotate(fn, meta) {
meta.superClass = fn.__superConstructor__;
fn.__TCmeta = meta;
return fn;
}
// export to global
__w = annotate;
/**
* when using the annotate function
*/
function sprintf(format) {
// function body
}
__w(sprintf, {"signature":"function(string)"}); // <-- what is the
purpose of doing this?
So, my question is what is that used for? What is the benefit doing this?