Note that setting a price alone doesn't lock down your book behind a paywall. It merely says that your book is priced. This means that you can price a book and still keep it open for everyone to read. In a hope that some of those readers will eventually pay you for the work.
This means that you can price a book and still leave it open for anyone to read! This kind of sales model is suitable for those who are new in town or are feeling charitable about readership and not necessarily money. Ofcourse some of those readers will happily pay, our experience says, if your book is good.
The second pricing tool locks down the content of your book at the page number set as page-marker. Sampling a chapter or two convinces a buyer better. Set a value to page-marker to ensure that the pages after page-marker are available only after a purchase has been made. This is more or less a standard model for all books