The change provide a user with the information about when the background multi-host operations is finished.
Some network changes (e.g. attach/detach labeled network, update network definitions) cause a background multi-host operation to be spawned. Whereas the user isn't aware of that background operation, he might initiate another network change that would spawn another background operation that would collide with the first one if that hasn't finished yet.
Make a user aware of the background processes that he starts, so he would avoid starting multiple processes that are prone to fail.