All writers want to submit the best book possible to their publisher, but there has to be a point in which the author believes the book is strong enough, well written enough to submit to that publisher or an agent.
The problem many writers have is letting a project go. We can revise a manuscript forever and in the process, edit the life right out of the story. I once knew a writer who revised her first chapter for months. She felt she couldn't move forward with the story until the first chapter was perfect. While there's nothing wrong with revising your work as you write it, but after 18 months that writer still had nothing written of her story except that first chapter. Her revisions became nothing more than changing a word here and there and then changing it back again.
I believe an author she admired told her a first chapter sells the rest of the story. She was so determined to write the first chapter perfectly that she became paralyzed by it and wasn't able to move forward. I'm not holding her up as an example of what not to do, but to show that all writers have to come to the decision that that last pass of edits and revisions is enough and to know that the draft is the best possible one at the time.