For example, if you graduate seniors to get graduation dates on their transcripts, but they needed to be added to Activities. Because Alumni cannot be enrolled into Activities, the class had to be un-graduated so the Users can be enrolled into Activities.
Or, if you need your Alumni to display in the Student Directory, and because they have been marked as Alumni they no longer display. If they are un-graduated, they can be moved back into the Student Role and re graduated with a Future Effective Date for the Alumni Role so they stay as Student in the Directory. 

Call, Chat, or Submit a Case through Case Central and let Support know that you need to un-graduate the Class of 20XX. We will get this request to the Data Team for completion and advise you when this is completed. Please note, depending on the time of year this request could take some time to complete as the Data Team/Support may be backlogged with requests due to end of year processes for Schools'.