At the time of writing, there are hundreds of plugins tagged with users in the WordPress plugin directory (https://wordpress.org/plugins/tags/users). We can divide those plugins into many groups, as the functionality offered is extensive. For example, there are plugins for the following:
- Dealing with various author accounts, co-authoring posts, and multi-author sites
- Constructing membership sites around WordPress where members can get access to premium packages of content based on their subscription model
- Building a classic e-commerce store where everyone can make a purchase from the available directory of things
- Creating an online forum based on WordPress
- Building an email newsletter sent to a given site's users directly from WordPress, instead of using external services
- Launching a social network on WordPress
- Managing user profiles for registered users
As you can see, the possibilities are striking. If we want to, we can do virtually anything with a WordPress site and its users. Only our imagination imposes a limit. That being said, you should always think about website security when considering any such plugins.