Numerous reasons can affect the availability of a product. Because you invested in a lot of effort to drive traffic to your store and get customers in front of your product, letting them leave just because you don't have a product in-store now cannot be an option.
With the back in stock feature, your clients can give you their contact details, and decide to be notified when it's available again either by email or SMS, depending on what they prefer. And the best part is that you don't need to do anything on your side. As soon as you add stock to that particular product, the app will automatically notify them.
What's also great is that you can use that feature to gain decisive insights on the products that your customers await most, and plan your restocks accordingly, so that you can get an idea of how many items you'll sell before you even order them.