Customers who become a "registered user" will have a permanent record of their name and mailing address on file. This will allow returning customers to checkout with their items more rapidly than a non-registered user. Registered users also have their previous shopping cart contents saved. This is especially useful to customers who need to make the same order on a regular basis.
You can see a list of your registered users under the "Customers" tab in EasyStoreMaker Pro's preferences.