Once the customer fills in the required details, an account for the customer will be created.

This is a convenience for returning customers who don't want to fill in their information every time they order a product.  And it's great for you, the merchant, because once they have registered, a customer's information will be recorded in your Customers list and will be available for you to send mailings to.  

NOTE: be sure to send marketing emails only to those registered customers who have opted in to join your mailing list.