Sometime a label is useful during a certain period of time. Despite a label is no longer applied to new messages, it is still important to be kept to tag old ones. For example, a messages related to a renovation you've made to your house in the past. The renovation is over, but you want to keep the history.

For the sake of cleanliness, It would be useful to be able to hide unused old label from the side bar. Messages tag with that label should still be found though.