The Payment Plan Payment event triggers actions after a successful payment plan payment is processed.

This is most commonly used for internal processes, to stop customer follow-up, and to send notifications. 

This event trigger is great to use for housekeeping. If you use tags to trigger all other Billing Automation Actions, after every successful payment you could make sure that the customer is removed from any billing follow-up sequences, and that tags are removed for things like Failed Payment, Subscription Unpaid, Card Expired, Card Updated, etc. You want to make sure that if those types of actions have been triggered on a contact record once, that they will fire again. Removing tags strategically is just as important as applying them strategically.

Also, you could use this in conjunction with your marketing automation tool, to trigger a special email after 6 months and 12 months of being subscribed. This would require getting creative with applying and removing tags to track and automate this process.