When the product on your store is out of stock, you will lose the business from the customer who reached that product to purchase. "Notify Me When this item is Back in Stock" will take customer email id and will store this mail address to the database. You will get email about the customer email address along with the product Name/URL for your acknowledgement.

Whenever you put that product back in stock, all the customers who submitted their email address towards that particular product, will get the automatically generated mail with the text like "Product back in stock, you can purchase." The product page link will also apply to the mail so that customer will get through the product page directly from this link. You will have the full control over the customer email address. You can either delete that customer email address or can store anywhere else for your future reference.

This feature will open a new way of customer relationship and will make your store alive. This functionality will reduce the email traffic at your mail account and it will make easier for you to manage long term relationship with customer.