The store builder provides the ability to create customer accounts. The customer account can be created by a store administrator or by the customer.
The main benefit to customers having accounts is that repeat customers will not have to fill in their billing and shipping information for each order. They enter the info once and then future purchases from your store are much easier.
If you have plans to offer special pricing to select groups of customers the customers will need to have accounts established to be able to be placed in a group.
To require customers to create a login:
1. Login to the administrator panel and select "General-->Login Settings".
2. Under the "Express Checkout" heading select the option labelled "Require all users to create a login and password".
3. Select the "Save changes" button