A good publisher can provide editing, formatting, graphics, advertising, and localization.
They can also add their stamp of approval to it (I know that this means nothing to a lot of buyers, but it does mean something to some. I have heard a couple of readers of mass fiction rave about trust Del Rey books to be worth reading because they trust Del Rey), and an established publisher can more easily get it into print if/when it is worth doing.
Of course an author can do all of those themselves or hire others to handle those steps for them. But it is certainly value-added to have all of that done for you.
They can also add their stamp of approval to it (I know that this means nothing to a lot of buyers, but it does mean something to some. I have heard a couple of readers of mass fiction rave about trust Del Rey books to be worth reading because they trust Del Rey), and an established publisher can more easily get it into print if/when it is worth doing.
Of course an author can do all of those themselves or hire others to handle those steps for them. But it is certainly value-added to have all of that done for you.