There aren't, in this example, any functional differences, but I do think the parentheses make it much more readable. How do you know how it is scoped without the parens? Is it hoisted? This is a brilliant thread on the subject.
ASIDE:
JSLint will complain that "function statements are not invocable. Wrap the whole function invocation in parens." I think most browser parsers not running in strict-mode would generally let it pass, but it's better not to rely on that.
