It's solely a syntactic thing in this case - so that you can invoke it as x.foo(y) rather than foo(x, y) or x.foo(x, y). So you need some magic for the receiver to become the function argument.

It can be argued that a better way to do this is to just have freestanding generic functions that dispatch on the first argument (or better yet, multimethods that dispatch on as many as you want), and then just consider x.f(y) to be a syntactic sugar for f(x, y). But generics themselves are fairly complicated, and many people don't like that extra complexity.