Promoting An IP
Let's say a customer is renumbering their IPs, for example from a /29 to a /28. On Cisco, the new /28 would be applied on their IP interface as a secondary subnet. Once the customer has migrated off the /29, it can be removed, and the /28 made the primary, without interruption to the customer. Just go into the interface, and set the IP as the /28 without specifying the "secondary" keyword. It will overwrite the /29 with the /28, and the customer will see no interruption.
This is also helpful when renumbering backbone links.