The Product Purchased event is only available for automation on the Product level and will trigger when a Product is purchased. Product level automation is available for Subscriptions and Single Products.
This event is most commonly used when selling the same Product across multiple checkouts and you want certain integration actions to trigger when the Product is purchased, regardless of what checkout the Product is on. You can set up the integration actions at the Product level so that you don’t need to set them up on each individual checkout.
example
I want to be sure that all my customers who purchase “Product A” will get a “Product A” tag in my CRM. These are actions that don’t trigger any customer-facing communication, but simply help me keep my database organized.
I would set these actions up using the Product Purchased event on the product-level, so I don’t have to take the additional steps to add them to every Checkout I create.