When we buy things like clothing or footwear, nothing can anger us as much as our size lacking in store offer. Because constant observation of the store is impossible (not to mention: rather tiresome), good on-line store should offer a special notification mechanism, which allows a customer to be informed about product availability. This mechanism often allows to increase sales. New version is not favorites-based (required to sign up to store and add product to the list of observed products), but allows to subscribe to a special one-time notification (which does not require to create account in store).
More information can be found at "E-mail and SMS notifications about product re-availability" page.