On the AppStore, about 10% of transactions have a billing issue and go through the “Billing Retry” status. This means that Apple could not charge the user for some reason (card expiration, not enough balance etc.) and is in the process of trying to charge the subscription amount.
This period takes 60 days and although some users are successfully charged and continue as subscribers, most of them are lost which is labeled “Involuntary Churn” by Apple.
With Grace Period, you give the customer the possibility to continue using your app even when the payment fails. This in turn, enables you to charge the user for those days if Apple manages to process the payment within your Grace period.
In summary, the Grace Period enables you to charge extra for a process that occurs naturally in the Apple ecosystem. Based on your Churn ratio and the number of “recovered” users within that Grace Period, the total amount can be quite considerable.
Please read more about Grace Period to know more