Marketing for an online business differs somewhat than for a business that has a street presence. Your web store cannot be seen when people go shopping in the CDB, but it may well show up when they shop online. Part of the marketing is in harvesting their email addresses so that you can establish a relationship with them.
Many businesses get email addresses simply by giving their customers the option of creating an account. In return, the consumer finds it easier to purchase what they want, especially the next time they shop there. The business can then send out marketing emails over the next few years or until/if they unsubscribe.