This environment is a nearly exact replica of the Production environment so it seeks to mirror an actual Production environment as closely as possible to ensure the software works correctly.
The staging environment is often restricted to a small number of selected users and so it is limited to certain IPs and developer teams.
The purpose of this environment is to test on a near-production level but in a non-Production environment to verify that the application will behave correctly after deployment. Thus, it represents a safe space away from the public eye to make sure everything works as expected; it is kept invisible from end-users.
The focus here is to test the application or software as a whole. Examples of the kind of test that can be run in this environment include Smoke Testing and in particular load testing since it can be difficult to replicate high volumes of traffic in a staging environment to test the software or application under stress.
However, keep in mind that a staging environment is not meant to be a substitute for a real-world Production environment.
To ensure your product works as it should, you need both a testing and staging environment, where the Testing environment makes sure that every component does its job while the staging environment ensures that your software would behave correctly in an environment almost exactly the same as production.