For when the customer pre-pays for multiple boxes and you want to receive email reminders for when to send their boxes out.
NB - You will also need to add the following custom fields for the product: pre_pay_item_quantity, pre_pay_despatch_interval_num, pre_pay_despatch_interval_units. For example, for a 12-month Vegan Tuck Box prepay product, these values would be:

pre_pay_item_quantity: 12
pre_pay_despatch_interval_num: 1
pre_pay_despatch_interval_units: monthly

(Permissible values for the pre_pay_despatch_interval_units custom field are: minutely, hourly, daily, weekly, monthly, annually.)