Think about any sort of private club or membership group. An elks club, a fan club, or a personal trainer’s own site that I mentioned above that sells access to on demand personal training video sessions and customized workout plans.
I actually know an HOA that uses IPB. The quarterly dues are paid through the website. Residents use the forums for discussion, but the pages application is used as a CMS to post meeting minutes, etc. It even has private resources for members vs guests, and separate resources that are just for the board. Commerce and Pages are PRIMARY applications for them honestly more than forums.
You’re thinking in term only of a forum when that’s not what the suite is. IPB’s roots was building forums, but now it builds communities. Different communities have different ways of monetizing itself. Ads are one way but certainly not the only way. Subscriptions… one time sales… micro transactions… there are lots of ways that could drive revenue! 🙂