Magento out of Stock Notification extension provides feature to keep tracking of "out of stock" products and customers who are waiting to purchase that products.
This is simple way; store owner will sell products quicker and save their time on unwanted items & unnecessary price modifications. It will improve the sales, by keeping your customers updated about the product stock availability.
The store owners can also request customers to enter their email id to get the update email when the product is back to stock. Admin can personalize the notification email by editing the default template in the admin panel OR admin can also create new as per needs.