This feature can be useful if you want to preview what your Moodle site will look like to students, for example.
In the course, go to the User Menu at top right and then select Switch Role To.
On the next screen Click on the role that you wish to assume (ie "student ac"). The course display will change based on what a person in that role would see.
To switch back to your original role, go to the User Menu and click the Return to my normal role link.
With some blocks, switching role doesn't always correctly change block behavior.