The visitor is really interested in a product but may not purchase it right now. But if the product is less expensive then they will buy. 

This automation notifies customers that the product price has been reduced, and helps remove their financial barriers to making a purchase.

This automation is very useful to convert such a visitor into an actual customer. 

Once you enable this automation, Price drop opt-in appears in storefront in all products. This price drop widget is fully customizable and it helps to take permission from visitors to send notification regarding price drop for that product.