When starting a project usually there are so many requests and ideas that you create ambitious Role concepts and rolling these out to all of your sites. After a while you see that not all features are really used. This was the point in time for me to get back to a more lean approach.
As any system, also your Sitecore installation will historically grow. And for sure, none of the users will tell you when he or she stops working on Sitecore or leaves the company. Also noone of the local administrators (in case you have some) will tell. So, to keep Sitecore a bit clean you can check for users that are not active in the system.