An action to modify the product, fee, shipping or discount line items on a subscription solves many different use cases we see requested via support.
Example use cases:
* provide subscribers with a discount on their subscription after the first 12 months (payment count > 12)
* add bonus/gift products to a specific renewal, like a reward for the 12 month subscribed
* create a "drip" like campaign, where customers receive a different product for each renewal, like a different volume of a book each month, or the next piece of a kit
* customer is given a discount from an unrelated trigger. E.g. customer leaves a review and get's $5 off their next renewal payment.
* stores could even use it to change tax line items in case of changes they need to complete with, like VAT Changes: https://docs.woocommerce.com/document/woocommerce-subscriptions-guide-vat/