It doesn't do what it should.
It sets a Reply-To: header, not a From: header.
Why, I have no idea. It is kind of stupid to just set a Reply-To: header. I can't imagine why they did that and not a From: header - mailing lists that rely on From: don't care what the Reply-To: header is, for crying out loud
Plus it does nothing to alleviate the problem people have of not being able to "impersonate" all of their email addresses.