Users who interact with the site can be awarded points. With enough points accumulated, a user becomes a member of a particular user group. You can restrict access to certain modules to registered users only; but you can take this even further, and allow only members of a particular user group access to the module. In this way, you can reward users who frequently participate in your community with extra modules or content that only they can view.
Although user groups and subscribed users seem similar in the sense that you can restrict module access exclusively for them, there are a number of key differences: