Who is your "Publisher"?

In reading about self-publishing I've seen a lot of people say you need to be your own publisher, buy the EIN numbers and list your own company name.  My question is, what does this get you?  CreateSpace and SmashWords are willing to be my "publisher" for free and get those pesky EIN number for me as free as well.  So what does opening a Publishing company buy me?  Is it worth the investment of a few hundred dollars?

Thanks for your help!

Pav