Hi Tarun,
When your page is in draft mode it will be visible only to authenticated users (or user who will have permisions on that site).
when you talk about public site, i belive it will be using annonimous authentication and users will not be authenticated and will not be able to see draft versins of pages. This is where publishing infrastructure comes in to picture to avoid anoonimous users
to see pages which are not published and approved.