Use case: I have a digital product and 3 upsells. Each upsell and the main product correspond to a module inside a course, so in total I have one course with 4 modules.
I want to be able to show the main product by default and if a user didn't buy upsells, I want him to see each module related to each upsell but they should be locked. A module can be unlocked upon purchase of an upsell related to this module.
Clickfunnels does this. This helps to organize nicely the content and earn more money.