A returning customer can be quickly processed even through phone and email order requests. Simply search for a previous order, either in the Manage Orders or through the Customers tab, open up that order and select Create Related Order from the action menu. This will generate a new order with that customer's information and shipping address.
Another benefit of related orders is to show relationship between two orders when shipping an item related to a previous order. For example, if a customer lost a part of a set and wants to order only that component, creating a related order will show the relationship between the two orders by referencing the order numbers next to each other.
Note: If you do not want to charge the customer for the related order you need to do it as a discount. Setting the item price to zero will not allow the order to ship