A very oversimplified explanation of a staged user:
A user that probably exists in real life but has never registered to your site but has somehow managed to create a topic (usually through the email in option)
The only use case for a staged user that I’m aware of is when someone has an email ID which is set up in discourse to allow topics in email and allowed the creation of a staged user account for any emails that do not have an account associated with discourse.
The benefit it has is that people who want to create a topic but don’t have an account can still send in their topics without going through the process of registering an account and then creating a topic.
Use case 1: anonymous posting from throwaway email (not sure how it’ll work but still)
Use case 2: creating a support email and using discourse to keep track of it (we use it)
Use case 3: Mailing list (again, Not sure)